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Adsorption, Equilibrium Isotherm, and Thermodynamic Studies towards the Removal of Reactive Orange 16 Dye Using Cu(I)-Polyaninile Composite

To overcome some of the limitations of activated carbon like efficiency, cost-effectiveness, and reusability, the present work deals with Cu(I)-based polyaniline (PANI) composite for the removal of reactive orange 16 (RO16) dye. Following the synthesis and characterization of formed Cu(I)-PANI composite, the batch experiments performed for the removal of RO16 dye indicated that the composite has the capacity to reduce the coloring from RO16. The experiments were conducted for the study of effects against changes in pH, time, and dose at room temperature, where we observed for a pH impact on the dye adsorption capacity in the range of 2–12. Among all, the optimal RO16 removal was found to be 94.77% at a pH of 4 and in addition, the adsorption kinetics confirmed to be pseudo-second-order with more suitability towards the Langmuir isotherm, where it is presumed to be the formation of a monolayer of dye molecule at the homogeneous absorbent surface. The calculated maximum capacity, qm, determined from the Langmuir model was 392.156 mg/g. Further application of isotherms to attain thermodynamic parameters, a slight positive value of ΔS° for RO16 adsorption was observed, meaning that there is an increased randomness in the irregular pattern at the specific Cu(I)-PANI interface for an adsorption process. This mechanism plays an essential role in maintaining the effects of water pollution; and, based on the analysis therefore, it is prominent that the Cu(I)-PANI composite can be employed as a promising and economical adsorbent for the treatment of RO16 and other dye molecules from the sewage in wastewater.


Introduction
In recent years, the increased demand to obtain pure drinking water with only limited resources has garnered interest in the development of new technologies for water purification and in upgrading the traditional water management processes. The usage

Materials
The chemicals used to synthesize Cu(I)-PANI, including aniline, methanol, and copper sulfate, were purchased from Sigma-Aldrich (Mumbai, India), as they are similar to Reactive orange 16 (RO16) dye. All the reagents used were of highest standards and are analytical grade; all the solutions were made using deionized water solvent.

Instrumentation
For the characterization of Cu(I)-PANI composite, the powder x-ray diffraction (XRD) was carried out on an XRD 6000 instrument (Shimadzu, Kyoto, Japan), and the UV-Vis spectroscopy analysis was studied using a SPECORD PLUS double beam spectrophotometer (Analytikjena, Jena, Germany). Also, the elemental analysis and their oxidation states was investigated using the X-ray photoelectron spectroscopy (XPS), and for that, AXIS 165 high performance multi-technique surface analyzer with a mean radius hemisphere of 165 mm and an eight channeltron detection system was used. The BET surface area was observed by mercury intrusion porosimetry (Model AutoPore IV 9500 V, Micromeritics, Norcross, GA, USA). The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were studied to investigate the thermal stability of the composite and for that, a Perkin-Elmer instrument was used.

Synthesis of Cu(I)-PANI Composite
The synthesis procedure used for the formation of Cu(I)-PANI composite was the slight modification of an earlier reported method elsewhere [19]. Briefly, 0.01 M of aniline solution was diluted in 25 mL of CH 3 OH in a conical flask followed by the addition of 0.01 M of CuSO 4 ·5H 2 O solution in distilled water, where the molar concentration ratio was maintained to 1:1 while stirring continuously for 24 h. First, the mixture at the time of addition was green in color and by the end of 24 h of stirring, the formation of parrot-green precipitation at the bottom of the flask can be noted. The complete reaction process was carried out at room temperature and at the normal atmospheric pressure. The material was filtered out and repeatedly washed using deionized water, and then dried at 50 • C for 24 h in a vacuum.

Preparation of Dye Solution
The RO16 dye was employed to test the adsorption capacity of the as-synthesized Cu(I)-PANI composite. For the testing, 0.5 g of dye powder was dissolved in 1000 mL distilled water to prepare a stock solution of 500 mg/L concentration. The diverse concentration of dye solution proceeded from the suitable dilution of stock solution with the distilled water.

Calibration Curve
The dilution of stock solution was used to prepare five samples of RO16 with different concentrations (20,40,60,80, and 100 mg/L). The maximum RO16 wavelength and the absorbance value of samples were taken into account by the UV-Vis spectra, and a calibration curve was established. The absorbance value received from the calibration curve of absorbance was 494 nm, and it was made at the maximum RO16 wavelength. According to IUPAC (International union of pure and applied chemistry) guidelines, the RO16 detection limit was 0.12 mg/L when using the spectrophotometric method [20].

Reusability
The cycles of adsorption-sorption were carried out repeatedly for five times to investigate the reusability of the synthesized composite and its adsorption capacity. The adsorbent was even reacted with 10 mL ethanol at room temperature. Figure 1a,b shows the comparison of the SEM images of Cu(I)-PANI composite at two different magnifications (scales of 2 µm and 100 nm) and from the figure, it can be noticed that the PANI surface is soft and porous.

Preparation of Dye Solution
The RO16 dye was employed to test the adsorption capacity of the as-syn Cu(I)-PANI composite. For the testing, 0.5 g of dye powder was dissolved in 1 distilled water to prepare a stock solution of 500 mg/L concentration. The diverse tration of dye solution proceeded from the suitable dilution of stock solution distilled water.

Calibration Curve
The dilution of stock solution was used to prepare five samples of RO16 wit ent concentrations (20,40,60,80, and 100 mg/L). The maximum RO16 wavelength absorbance value of samples were taken into account by the UV-Vis spectra, an bration curve was established. The absorbance value received from the calibratio of absorbance was 494 nm, and it was made at the maximum RO16 wavelength. ing to IUPAC (International union of pure and applied chemistry) guidelines, t detection limit was 0.12 mg/L when using the spectrophotometric method [20].

Reusability
The cycles of adsorption-sorption were carried out repeatedly for five times tigate the reusability of the synthesized composite and its adsorption capacity. Th bent was even reacted with 10 mL ethanol at room temperature.   These surface-porous sites may serve as the stable houses for the localization of adsorbent molecules when the Cu(I)-PANI composite is employed for adsorption-related applications. Further, the TGA was used for testing the thermal stability of Cu(I)-PANI composite where the material was kept to rest at 30 • C for 1 min, and, after that, it was heated between 30 and 600 • C at 10 • C/min ( Figure 1c). As shown in the figure, there is a loss of mass to the Cu(I)-PANI composite when heated in an atmosphere of nitrogen; we observed a weight loss to the Cu(I)-PANI composite at first below 298 • C, referring to the removal of physisorbed water and dopant ions. The subsequent loss observed in the range of 300 to 590 • C corresponds to the degradation of the polymer backbone. This weight loss can be assigned to the expulsion of water from the structure of the polymer, but at a higher temperature, the degradation of the polymer chain occurs because of the dopant. On dopant removal, the decomposition of polymer skeleton takes place at an extreme temperature [21]. The weight loss of composite above 590 • C temperature was only 1.28%, meaning that the Cu(I) addition is restricting the degradation of PANI. Based on the analysis, the TGA shows a low thermal stability of Cu(I)-PANI composite because of the inculcation of Cu(I) particles in the form of dopant at the N 2 sites of PANI [22].

Physicochemical Studies
DSC can be employed to study the glass transition and melting behavior of any sample. Figure 1d shows the DSC analysis of Cu(I)-PANI composite. From the graph, it is observed that the PANI shows only one endothermic peak at 85 • C, which is mainly due to the vaporization of water added to the glass transition behavior [23]. The importance of the glass transition temperature on the assimilation of the inorganic phase in the polymer matrix by in-situ polymerization is associated with excellent interfacial interaction between the particles and polymer, causing a restriction on the mobility of the PANI chain. Also, the graph shows a weak endothermic dip at 260 • C temperature, which could be from the melting of the PANI chain. It can also be observed that the onset of thermal degradation at a low temperature along with doping exhibits that the metal cations that are adsorbed on the surface of the molecule may be affected very quickly. This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.
The XRD is non-destructive, and it is an excellent technique for identifying the crystallinity of solid materials; the powdered XRD of Cu(I)-PANI composite is shown in Figure 2a, where the diffraction patterns of PANI are observed at 2θ of 13.08 • , 19.3 • , and 25.2 • [24]. Similarly, the Cu's diffraction patterns are observed around 43.3 • , 51.1 • , and 75.1 • corresponding to the plans of (111), (200), and (311) respectively, indicating for the presence of particles in the face-centered cubic lattice crystals [25]. It can also be noticed for the non-observation of CuSO 4 peaks in the XRD pattern, meaning that the Cu particles are reduced during the polymerization process. The XPS analysis for the Cu(I)-PANI composite is provided in Figure 3(a) where the survey spectrum confirming the presence of Cu element along with other elements of C, O, and N from the PANI polymer. In addition, the elemental Cu 2p spectrum provided in the inset is indicating for the presence of two different peaks, one at 932.2 eV for the Cu 2p3/2 and the other at 952.2 eV for the Cu 2p1/2 orbital confirming that the Cu is in its +1 oxidation state only [19,27]. Thus, from the XPS analysis, it can be confirmed for the suc-  Figure 2b shows the UV-Vis spectroscopy of Cu(I)-PANI composite, as this is a very essential technique for the characterization of interfacial interactions between CuSO 4 and PANI. From the spectrum, two different absorption peaks can be observed in the wavelength range of 320-330 nm and 600-660 nm which can be linked to the corresponding transitions of π-π* and n-π* (respectively) of benzenoid and quinoid rings [26]. In particular, the peak observed at 325 nm refers to the π-π* transition of benzenoid, and 620 nm for the quinoid unit with a blue shift indicating the availability of Cu chromophores. However, the surface plasmon absorption (SPA) of Cu particles is 500 nm which is absent in the optical absorption of the Cu(I)-PANI composite. This could be mainly due to the presence of Cu content, which is less and has a strong absorption of PANI at a 600-660 nm range, overlapping the SPA of Cu particles.
The XPS analysis for the Cu(I)-PANI composite is provided in Figure 3a where the survey spectrum confirming the presence of Cu element along with other elements of C, O, and N from the PANI polymer. In addition, the elemental Cu 2p spectrum provided in the inset is indicating for the presence of two different peaks, one at 932.2 eV for the Cu 2p3/2 and the other at 952.2 eV for the Cu 2p1/2 orbital confirming that the Cu is in its +1 oxidation state only [19,27]. Thus, from the XPS analysis, it can be confirmed for the successful formation of Cu-PANI composite with the oxidation state of Cu to be +1, but not in +2.

RO16 adsorption studies
Solution pH is a significant factor for ionization dissolved in a solution, a surface, dye molecular structure, and for the separation of various functional g active sites [30]. Figure 4 shows the comparison of pH effect towards the removal The BET isotherm of Cu(I)-PANI composite is shown in Figure 3b, simulating Type IV isotherm to the hysteresis loop of type H3 dependent on IUPAC classification [28]. It implies that the formed polymer composite material has a large aperture and also contains porosity. The adsorption-desorption isotherms of material are responsible for showing the type H3 hysteresis loop on it, inferring that the capillary condensation is occurring along with the mesopores. The surface area value to the Cu(I)-PANI composite is 20.5 m 2 /g, total pore volume of 0.15 cm 3 /g, and an average pore diameter of 25.34 nm was calculated. Such an observation of values refers to the fact that the Cu(I)-PANI composite has a mesoporous structure.
The BET adsorption analysis exhibited that the pore size distribution has an inverse relationship between pore diameter and surface-area density. The pore distribution of material clarifies that the surface is engrossed by a comparatively more minor mesopore structure that ranges between 7 and 20 nm (as shown in Figure 3c). The pore volume also shows a similar pattern to that of pore size since the high surface area and porosity influences the composite's performance; and so, these are considered to be the significant parameters of solidstate materials [29].

Effect of pH
Solution pH is a significant factor for ionization dissolved in a solution, adsorbent surface, dye molecular structure, and for the separation of various functional groups on active sites [30]. Figure 4 shows the comparison of pH effect towards the removal of RO16 dye in the range of 2-12.

Effect of pH
Solution pH is a significant factor for ionization dissolved in a solution, adsorbent surface, dye molecular structure, and for the separation of various functional groups on active sites [30]. Figure 4 shows the comparison of pH effect towards the removal of RO16 dye in the range of 2-12. From the graph, the removal efficiency of RO16 is raised by moving the pH from 2 to 4, and tends to be decreased beyond pH 4. At this pH, the maximum % removal for the RO16 dye was observed to be 94.77% and so, all the successive experiments on the From the graph, the removal efficiency of RO16 is raised by moving the pH from 2 to 4, and tends to be decreased beyond pH 4. At this pH, the maximum % removal for the RO16 dye was observed to be 94.77% and so, all the successive experiments on the adsorption process were performed at an optimal pH of 4. Further, when the pH is increased from 5 to 12, the % removal of RO16 dye is decreased. This tendency can be mainly due to the competition between extra OH − ions and negatively charged dye molecules. In reality, since the pKa of RO16 is 3.75, the anionic species were the preponderant RO16 species in the solution at pH levels above this value. The most significant amount of RO16 was adsorbed at pH 4, which had a q e 157.95 mg/g, followed by pH 12, which had a q e 128.55 mg/g and was the lowest.
The point of zero charges (PZC) can be formed when the pH surface charge tends to zero, and such PZC is employed to define the surface's electrokinetic features. The pH pzc value is 4.92. The pH value is applied for describing the PZC of the system, where H + /OH − is performing its potentials in determining the ions. The anionic dye adsorption process is favored at pH<pH pzc because of the availability of functional group OH − and the surface tends to become positively charged [31].

Effect of Dose
The critical aspect of adsorbent dosage determines the adsorbent capacity and in that view, the Cu(I)-PANI composite dose in the range of 0.01 to 0.1 g/L was tested at a concentration of 100 mg/L dye solution to measure the % removal of RO16, as shown in Figure 5.
reality, since the pKa of RO16 is 3.75, the anionic species were the preponderant RO16 species in the solution at pH levels above this value. The most significant amount of RO16 was adsorbed at pH 4, which had a qe 157.95 mg/g, followed by pH 12, which had a qe 128.55 mg/g and was the lowest.
The point of zero charges (PZC) can be formed when the pH surface charge tends to zero, and such PZC is employed to define the surface's electrokinetic features. The pHpzc value is 4.92. The pH value is applied for describing the PZC of the system, where H + /OH − is performing its potentials in determining the ions. The anionic dye adsorption process is favored at pH<pHpzc because of the availability of functional group OH − and the surface tends to become positively charged [31].

Effect of Dose
The critical aspect of adsorbent dosage determines the adsorbent capacity and in that view, the Cu(I)-PANI composite dose in the range of 0.01 to 0.1 g/L was tested at a concentration of 100 mg/L dye solution to measure the % removal of RO16, as shown in Figure  5. From the graph, it is easy to understand an increase in the % removal of RO16 dye with that of increased adsorbent dose until 0.06 g/L, and this can be linked to the availability of a higher number of adsorbing sites in the composite. However, with further increase in the adsorbent dose beyond 0.06 g/L, no significant increase in the sorption is observed and may be the reason that the sorption amount becomes constant due to accumulation of adsorbents.
From the study, it was observed that the RO16 % removal efficacy tends to be increased at the optimum dose, and after that, there was no change in the efficiency. This could be mainly due to the fixed initial concentration of solute or an increase in the adsorbent dosage, leading to an increased surface area and linked adsorption sites [32]. When the adsorbent concentration is least, the surface becomes saturated by the dyes, where the residual dye concentration tends to become highest. Therefore, it was observed that the optimal dose for the adsorption of RO16 was 0.06 g/L, which was taken into account for carrying out further experiments.

Effect of Initial Concentration and Contact Time
The contact time is considered as a significant aspect in adsorption studies, as it provides information about the kinetics of total interaction occurring at the surface of the From the graph, it is easy to understand an increase in the % removal of RO16 dye with that of increased adsorbent dose until 0.06 g/L, and this can be linked to the availability of a higher number of adsorbing sites in the composite. However, with further increase in the adsorbent dose beyond 0.06 g/L, no significant increase in the sorption is observed and may be the reason that the sorption amount becomes constant due to accumulation of adsorbents.
From the study, it was observed that the RO16 % removal efficacy tends to be increased at the optimum dose, and after that, there was no change in the efficiency. This could be mainly due to the fixed initial concentration of solute or an increase in the adsorbent dosage, leading to an increased surface area and linked adsorption sites [32]. When the adsorbent concentration is least, the surface becomes saturated by the dyes, where the residual dye concentration tends to become highest. Therefore, it was observed that the optimal dose for the adsorption of RO16 was 0.06 g/L, which was taken into account for carrying out further experiments.

Effect of Initial Concentration and Contact Time
The contact time is considered as a significant aspect in adsorption studies, as it provides information about the kinetics of total interaction occurring at the surface of the adsorbent with that of the adsorbent. The contact time of dye adsorption for 240 min at an initial concentration of 100 mg/L and optimal pH is shown in Figure 6.
From the figure, the time variation plots clarify that the dye removal efficiency is the starting stage, but there is a gradual slow down in the removal process when it reaches a stage of equilibrium. It is mainly because of the vacant surface sites at the preliminary stage. After some time, the vacant sites become occupied by the dye molecules, creating repulsive force in between the adsorbate and bulk phase. The equilibrium is achieved when the solution is agitated with adsorbent for 90 min. Moreover, the equilibrium showed no change following the time. Such an action means that once the equilibrium is attained, there is no chance of removing any treatment. adsorbent with that of the adsorbent. The contact time of dye adsorption for 240 min at an initial concentration of 100 mg/L and optimal pH is shown in Figure 6. From the figure, the time variation plots clarify that the dye removal efficiency is the starting stage, but there is a gradual slow down in the removal process when it reaches a stage of equilibrium. It is mainly because of the vacant surface sites at the preliminary stage. After some time, the vacant sites become occupied by the dye molecules, creating repulsive force in between the adsorbate and bulk phase. The equilibrium is achieved when the solution is agitated with adsorbent for 90 min. Moreover, the equilibrium showed no change following the time. Such an action means that once the equilibrium is attained, there is no chance of removing any treatment.
In batch adsorption, the removal rate of adsorbate is controlled through the transportation of dye molecules present in the nearby sites to the internal sites of particles [33]. Hence, from the above context, the amount of absorbed dye changes with the variation of initial dye concentration, and increases with that of an increased dye concentration. As the equilibrium is attained, the amount becomes constant because the driving force is offered from a rise in the solute concentration, enabling it to overcome the resistance towards mass transfer between solid and liquid phases. Therefore, the adsorption tends to increase at higher initial concentrations. As the initial dye concentration increased from 20 to 100 mg/L at a temperature of 318 K and pH 4.0 ± 0.1, the adsorption capacity also increased from 31.55 to 157.79 mg/g. At the same time, the %removal of RO16 decreased from 94.67% to 38.2%. It is also observed that the fractional removals are high at high concentration and at low concentration, and the removal of dye is high [34].

Adsorption Isotherm
The adsorption isotherm elaborates the relation between the amount of adsorbate taken up and the concentrated adsorbate of the remaining solution. Various equations are presented for analyzing the experimental adsorption equilibrium data. Such parameters for the equilibrium model give insight into the adsorption mechanism, features of the surface, and adsorbent affinity [35]. For the study, the Langmuir and Freundlich isotherm models are analyzed to understand which one fits well.
The isotherm achieved for the adsorption of RO16 dye on Cu(I)-PANI composite is shown in Figure 7a,b. In batch adsorption, the removal rate of adsorbate is controlled through the transportation of dye molecules present in the nearby sites to the internal sites of particles [33]. Hence, from the above context, the amount of absorbed dye changes with the variation of initial dye concentration, and increases with that of an increased dye concentration. As the equilibrium is attained, the amount becomes constant because the driving force is offered from a rise in the solute concentration, enabling it to overcome the resistance towards mass transfer between solid and liquid phases. Therefore, the adsorption tends to increase at higher initial concentrations. As the initial dye concentration increased from 20 to 100 mg/L at a temperature of 318 K and pH 4.0 ± 0.1, the adsorption capacity also increased from 31.55 to 157.79 mg/g. At the same time, the %removal of RO16 decreased from 94.67% to 38.2%. It is also observed that the fractional removals are high at high concentration and at low concentration, and the removal of dye is high [34].

Adsorption Isotherm
The adsorption isotherm elaborates the relation between the amount of adsorbate taken up and the concentrated adsorbate of the remaining solution. Various equations are presented for analyzing the experimental adsorption equilibrium data. Such parameters for the equilibrium model give insight into the adsorption mechanism, features of the surface, and adsorbent affinity [35]. For the study, the Langmuir and Freundlich isotherm models are analyzed to understand which one fits well.
The isotherm achieved for the adsorption of RO16 dye on Cu(I)-PANI composite is shown in Figure 7a Also, the isotherm constants and correlation coefficients are estim therm model, and the outcome of such a result is tabulated in Table 1. Also, the isotherm constants and correlation coefficients are estimated for the isotherm model, and the outcome of such a result is tabulated in Table 1. From the comparative study of the linear plots of Langmuir and Freundlich models, it was observed that the R 2 value of Langmuir isotherm model is 0.996, while Freundlich isotherm was 0.982 (Table 1). These R 2 values indicate that the adsorption process is observed to fit well with the Langmuir isotherm model, and this also suggests that the adsorption possesses continuous adsorption energy. In other words, the Langmuir isotherm goes well with experimental data mainly because of the homogeneous distribution of active adsorption sites on the Cu(I)-PANI surface as the surface is homogeneous [36]. The maximum adsorption capacity was found to be 392.156 mg/g. The Freundlich constant, 1/n is smaller than 1, showing that the adsorption process can be suitable to apply at optimum conditions [37].

Kinetic Studies
Adsorption kinetics is responsible for measuring the adsorption method efficiency through different applications of the kinetic model. The non-linear pseudo-first-order (PFO) is employed to determine the adsorption mechanism of dye molecules on the adsorbent surface, whereas the pseudo-second-order (PSO) can be applied to test the experimental data of initial concentration as exhibited in Figure 8. The PSO is responsible for expressing the adsorption method controlled by the chemisorption, whereas in PFO, the adsorption process is explicitly controlled by the physisorption [38].
Also, the isotherm constants and correlation coefficients are estimate therm model, and the outcome of such a result is tabulated in Table 1. From the comparative study of the linear plots of Langmuir and Freun it was observed that the R 2 value of Langmuir isotherm model is 0.996, wh isotherm was 0.982 (Table 1). These R 2 values indicate that the adsorption served to fit well with the Langmuir isotherm model, and this also sugges sorption possesses continuous adsorption energy. In other words, the Lang goes well with experimental data mainly because of the homogeneous dist tive adsorption sites on the Cu(I)-PANI surface as the surface is homogen maximum adsorption capacity was found to be 392.156 mg/g. The Freund 1/n is smaller than 1, showing that the adsorption process can be suitable t timum conditions [37].

Kinetic Studies
Adsorption kinetics is responsible for measuring the adsorption met through different applications of the kinetic model. The non-linear pseu (PFO) is employed to determine the adsorption mechanism of dye molecu sorbent surface, whereas the pseudo-second-order (PSO) can be applied to imental data of initial concentration as exhibited in Figure 8. The PSO is r expressing the adsorption method controlled by the chemisorption, where adsorption process is explicitly controlled by the physisorption [38]. The kinetic data were following the application of both the non-linear form of PFO and PSO and the obtained parameters are listed in Table 2. For linearized PFO, it was found that the results diverge from the straight line indicating that pore diffusion is not a controlling step for individual rate. The alteration of the R 2 value from unity is a measure of incompatibility. At present, the PFO exhibits poor R 2 . As shown in Table 2, it was observed that the adsorption of RO16 by Cu(I)-PANI is following the PSO in a higher R 2 value. This shows that the adsorption is a strong dependent of the presence of vacant adsorbent sites despite initial dye concentration. The PSO data make it clear that the adsorbate mechanism and adsorbent interaction coordinate with the physical attachment and the adsorption process is controlled by chemosorption. Table 2. Kinetic parameters for RO16 dye adsorption onto Cu(I)-PANI composite.

Thermodynamics
For demonstrating the feasibility of RO16 dye adsorption onto Cu(I)-PANI composite, the thermodynamic study of the adsorption process can be conducted. The values of ∆H • and ∆S • are measured from the slope and intercept of Van't Hoff plot (lnq e /C e vs. I/T), as shown in Figure 9 and the values calculated are tabulated in Table 3. and PSO and the obtained parameters are listed in Table 2. For linearized PFO found that the results diverge from the straight line indicating that pore diffusion controlling step for individual rate. The alteration of the R 2 value from unity is a of incompatibility. At present, the PFO exhibits poor R 2 . As shown in Table 2, it served that the adsorption of RO16 by Cu(I)-PANI is following the PSO in a h value. This shows that the adsorption is a strong dependent of the presence o adsorbent sites despite initial dye concentration. The PSO data make it clear tha sorbate mechanism and adsorbent interaction coordinate with the physical att and the adsorption process is controlled by chemosorption.

Thermodynamics
For demonstrating the feasibility of RO16 dye adsorption onto Cu(I)-PANI site, the thermodynamic study of the adsorption process can be conducted. The v ΔH° and ΔS° are measured from the slope and intercept of Van't Hoff plot ( ln I/T), as shown in Figure 9 and the values calculated are tabulated in Table 3.    From the results, the negative values of ∆H • confirm the fact that the dye adsorption method is exothermic and physical. There is confirmation of firm bonding on the adsorbent, which facilitates adsorbate molecules located on adsorption sites for an extended period since the molecular geometry hampers the slip-off mechanism from the adsorbent surface [39]. The negative values of ∆G • exhibit the feasibility and spontaneous nature of the adsorption process. In other words, the increase in a negative value of ∆G • decreases the temperature exhibiting the quantity adsorbed during the equilibrium, and it decreases with an increase in the temperature [40]. The value of ∆G • for the physisorption ranges between −20 and 0 kJ/mol, and the chemisorption process ranges from −80 to −400 kJ/mol [41]. For the corresponding study, the value of ∆G • is in the range of −20 to 0 kJ/mol, demonstrating that the process favors for a physisorption mechanism.
The ∆H • has a positive value showing that the adsorption process is endothermic and it was also observed that the value of % removal increases with an increase of temperature from 298 to 323 K. This also confirms that the process is endothermic. The positive value of ∆S • established the affinity and random increase of solid solution for the adsorption of dye on the Cu(I)-PANI composite.

Reusability
It is significant to regenerate and reuse the absorbent once it becomes saturated by dyes for practical uses. The eluent should possess different features such as activity, not be harmful to the adsorbent, non-polluting towards the atmosphere, and of course it should be cost-effective. For practical purposes, it must have two crucial factors: stability and reusability, which should be monitored thoroughly. Therefore, in the above context, the material's reusability investigation was performed to determine the effect of material on % removal when the usage cycle is carried out repeatedly. Figure 10 shows the reusability of material for adsorption of RO16 for the five corresponding cycles. bent, which facilitates adsorbate molecules located on adsorption sites fo period since the molecular geometry hampers the slip-off mechanism from surface [39]. The negative values of ΔG° exhibit the feasibility and spontan the adsorption process. In other words, the increase in a negative value of the temperature exhibiting the quantity adsorbed during the equilibrium, a with an increase in the temperature [40]. The value of ΔG° for the physis between −20 and 0 kJ/mol, and the chemisorption process ranges from −80 [41]. For the corresponding study, the value of ΔG° is in the range of −2 demonstrating that the process favors for a physisorption mechanism.
The ΔH° has a positive value showing that the adsorption process is en it was also observed that the value of % removal increases with an increase from 298 to 323 K. This also confirms that the process is endothermic. The of ΔS° established the affinity and random increase of solid solution for the dye on the Cu(I)-PANI composite.

Reusability
It is significant to regenerate and reuse the absorbent once it become dyes for practical uses. The eluent should possess different features such be harmful to the adsorbent, non-polluting towards the atmosphere, an should be cost-effective. For practical purposes, it must have two crucial fa and reusability, which should be monitored thoroughly. Therefore, in the the material's reusability investigation was performed to determine the eff on % removal when the usage cycle is carried out repeatedly. Figure 10 sh bility of material for adsorption of RO16 for the five corresponding cycles.  Figure 10, it is observed that there is a minute decrease in th after each cycle. The % removal of RO16 for the second, third, fourth, and 91.25%, 87.5%, 85.8%, and 81.2%. Such an observation of slight decrease is m of the substantial weight loss that took place at the time of recovery and d tion of the adsorbent. It also leads to a decrease in the % efficiency in every r and, in addition, there was a reduction in active sites present during the pr the dye molecules that were adsorbed. The investigation found that nearly was removed in the fifth cycle, which is a significant amount, thereby higher adsorption efficiency of Cu(I)-PANI composite. The stability and important aspects of an ideal adsorbent, as they help to reduce the cost of cedures, adsorbent supply, and help to minimize the adsorbent's disposal From the Figure 10, it is observed that there is a minute decrease in the productivity after each cycle. The % removal of RO16 for the second, third, fourth, and fifth cycles are 91.25%, 87.5%, 85.8%, and 81.2%. Such an observation of slight decrease is mainly because of the substantial weight loss that took place at the time of recovery and during purification of the adsorbent. It also leads to a decrease in the % efficiency in every repeated cycle; and, in addition, there was a reduction in active sites present during the process owing to the dye molecules that were adsorbed. The investigation found that nearly 81% of RO16 was removed in the fifth cycle, which is a significant amount, thereby indicating the higher adsorption efficiency of Cu(I)-PANI composite. The stability and reusability are important aspects of an ideal adsorbent, as they help to reduce the cost of treatment procedures, adsorbent supply, and help to minimize the adsorbent's disposal problem. The reusability test of materials was conducted by evaluating the RO16 adsorption efficiency after treatment with four different eluents (dH 2 O, HNO 3 , (CH 3 ) 2 CO, and C 2 H 5 OH). The Cu(I)-PANI stability was evaluated by measuring the concentration of Cu leachate after each adsorption/desorption cycle. From the analysis, the concentration of Cu after the fifth cycle was measured to be 0.088 mg/L, which is a negligible amount. This suggests that the Cu(I)-PANI composite has good stability towards the sequestration of RO16 dye in aqueous solutions. Finally, the analysis provides superior efficiency, stability, and reusability factors for the Cu(I)-PANI composite towards the adsorptive removal of RO16 dye that can additionally be tested for other novel dyes of long chain originics.

Comparison with Other Adsorbents
The comparative study of the adsorption capacity of RO16 was conducted with Cu(I)-PANI composite for different adsorbents as shown in Table 4. The outcome displayed maximum adsorption capacity for the study as compared to the studies conducted with other adsorbents as reported in the literature studies [42][43][44][45][46][47][48][49][50]. Thus, the conclusion can be made that Cu(I)-PANI composite has a relatively higher adsorption capacity, indicating that it can be recognized as a suitable material to remove RO16 dye from the aqueous solutions.

Conclusions
In conclusion, we indicate the high adsorption capacity, stability, and cost-effective nature of Cu(I)-PANI composite towards the removal of RO16 dye. As per the qualitative study, it can be mentioned that the Cu(I)-PANI possesses some extraordinary ability to remove the dye from aqueous solutions as compared to the other adsorbent materials. Following the synthesis and physicochemical characterization studies, the testing of Cu(I)-PANI composite confirmed its active adsorption behavior and so serves as an alternative to the activated carbons, and also, the composite does not possess any harmful effects. With the Cu(I)-PANI composite, the highest % removal observed was 94.77% at a pH 4, maximum adsorption capacity of 392.156 mg/g, and tended to be efficient at a low cost. The Langmuir and Freundlich's isotherms were used to analyze the equilibrium data and provided correlation for the adsorption of RO16 dye. The graph of linear equations was also used to determine the characteristic parameters for the isotherm and the correlation coefficient. The thermodynamic parameters indicated that the outcome of RO16 adsorption is spontaneous and exothermic. Based on the thermodynamic feasibilities and efficiency of