Studies on copper (II) removal from aqueous solutions by poly (3,4-ethylene dioxythiophene) polystyrene/sulphonate Sn (IV)tungstatophosphate (PEDOT: PSS/STP) nanocomposite

ABSTRACT To address the pollution of heavy metal ions in water resources, a new adsorbent poly (3,4-ethylene dioxythiophene) polystyrene/sulphonate Sn (IV)tungstatophosphate (PEDOT: PSS/STP), made of organic and inorganic materials, was effectively produced using a sol-gel bottom-up approach. The surface morphology, elemental composition and, structural features of the synthesised nanocomposite were investigated by various spectroscopic techniques such as XRD, TEM, FESEM, EDAX, and FTIR, whereas the surface charge measurement (Zeta potential) and ion exchange capacity (IEC) were determined by dynamic light scattering (DLS) analysis and standard column method, respectively. The batch experiment technique was utilised to determine the parameters for analysing the adsorption behaviour for the elimination of Cu (II) ions from aqueous solution. The kinetics of the adsorption process were investigated using pseudo-first-order and pseudo-second-order kinetic models, while the thermodynamics were assessed using Langmuir, Freundlich, Dubinin-Radushkevich (D-R) and Temkin thermodynamic models. The Langmuir isotherm was best fit to experimental data for adsorption of Cu (II) ions from an aqueous solution with a maximum adsorption capacity of 85.05 mg g−1 at pH 7.0 at different temperatures, whereas kinetic adsorption characteristics were best suited to the pseudo-second-order model. The thermodynamic parameters ΔHo, ΔGo, and ΔSo were determined and represent Cu (II) adsorption onto PEDOT: PSS/STP is an endothermic, spontaneous, and chemisorption reaction. Furthermore, even after five adsorption-desorption cycles, the adsorption capacity of PEDOT: PSS/STP nanocomposite adsorbent was around 70%, demonstrating that the regeneration capability of PEDOT: PSS/STP nanocomposite adsorbent was excellent, which is the fundamental characteristic to select the proper adsorbent.


Introduction
The carcinogenic organic and inorganic pollutants have become one of the challenging issues for ecology and the environment in recent years, which needed to be treated effectively, due to their discharge in surface and groundwater bodies particularly in developing nations [1][2][3].Cr (II), Ni (II), Cu (II), Hg (II), Pb (II), Cd (II), and Zn (II) are some of the heavy metal pollutants that are dumped into water bodies regularly; among them, Cu (II) is one of the most important impurities.Cu (II) in drinking water is highly harmful to all types of living beings at greater concentrations, over the allowed level, according to WHO, and the acceptable limit of Cu (II) in drinking water is only 1.3 mg/L [4,5].The major causes of heavy metal in contaminated water bodies include industrial water, pulp and textile factories, metal cleaning and plating baths, wood fibre production, and fertiliser companies [2].Cu (II) can enter the human body through dust, water, and nutrition, depositing in the liver, which may cause disorders including abdominal discomfort, while prolonged exposure may bring renal and hepatic damage [6].Nausea, stomach pain, and muscle soreness are all signs of Cu (II) poisoning.Excessive Cu (II) accumulation in hepatic parenchyma cells, the kidneys, the periphery of the iris, and the brain, produces catastrophic illnesses such as Wilson's disease [7,8].Hence, removal of these toxic metals from the contaminated water bodies becomes essential to preserve the biological life on the earth.
Up to date, a range of strategies for eliminating heavy metal ions from wastewater bodies has been explored.Adsorption, solvent extraction, nanofiltration, reverse osmosis, ion exchange, and chemical precipitation are just a few of the ways exploited to remove Cu (II) from aqueous systems [9][10][11].Adsorption is one of the most environmentally beneficial and cost-effective techniques for disinfecting water bodies among these options [12].All of the approaches outlined above urge towards the construction of polymer-based systems that interact with water for regular activities for the treatment of wastewater for several contaminants as experts attempt to boost the favourable surface contact between water bodies and polymers.To achieve environmental sustainability, polymeric materials provide an alternative for providing surface hydrophilicity and good biocompatibility [13].
The organic-inorganic hybrid nanocomposites are mainly synthesised by the bottom-up sol-gel approach.For the synthesis of nanocomposites, the sol-gel approach requires low temperatures at which the organic materials survive to synthesise the nanocomposite hybrid materials, which have unique properties as compared to their bulk counterparts [14].The hybrid organic-inorganic materials prepared by this method have gained tremendous attention from the last few decades particularly in physics, ceramics, polymer, inorganic, and organic chemistry fields [15].The nanocomposites exhibit novel and distinct qualities when compared to the traditional macroscale composites, which are due to their particle size differences.The sol-gel chemistry helps the inorganic nanomaterial to alter its properties by the incorporation of conducting polymer (CP), into a different nanocomposite material.The advanced multifunctional nanomaterials synthesised may have huge impacts on future applications in distinct fields such as thermal, structural, optical, electrical, sensors, membranes, catalysis, adsorption, and many more.The fundamental benefit of sol-gel chemistry is that it allows organic species to combine with inorganic precursors, resulting in composite or hybrid systems.Specifically, the in-situ sol-gel incorporation of inorganic particles within a polymer matrix enables good dispersion [16][17][18][19].Researchers have been paying a lot of attention to hybrid organic/inorganic nanocomposites recently, whether from a practical or basic standpoint [18,20,21].
The conducting polymers are advantageous over the traditional inorganic counterparts because of their low thermal conductivity, low density, good flexibility and portability, and tremendous applications such as photovoltaics, light-emitting diodes, thermoelectric, sensors, water splitting catalysis, etc [22][23][24][25].There are various CP-based nanocomposites such as PEDOT: PSS, polypyrrole, polyaniline, poly(3-hexylthiophene) and have excellent properties upon formation of their nanocomposites with inorganic additives [22,[26][27][28][29][30][31].PEDOT is among the most useful conducting polymers because of its high transparency, flexibility and conductivity, and remarkable environmental and thermal resistance.The CP, PEDOT: PSS comprises of mainly two parts; one is highly conductive hydrophobic PEDOT and the second is an insulating and hydrophilic PSS chain.PEDOT is a brilliant conducting fibre and can be used as an additive material in many fields such as flexible sensors, thermoelectric materials, etc., due to its exceptionally high conductivity and environmental stability.The solubility of the hydrophobic PEDOT part has been increased by the incorporation of hydrophilic PSS [poly (styrene sulphonate)] which acts as a charge balancing dopant [23,32].
The nanocomposite PEDOT: PSS/STP and inorganic part (STP) has been prepared by a simple bottom-up sol-gel approach and their formation were confirmed by various characterisation techniques such as FTIR, TGA, DTA, XRD, FESEM, EDAX, and TEM.The Zeta potential and IEC of both the inorganic counterpart and the nanocomposite were determined and shows excellent results.TGA was used to test the thermal stability of both the organic/inorganic hybrid nanocomposite (PEDOT: PSS/STP) and the inorganic counterpart (STP), the nanocomposite demonstrates exceptional thermal stability.However, there is no study regarding the adsorption of Cu (II) onto the PEDOT: PSS/STP nanocomposite.This study aimed to explore the adsorption behaviour of PEDOT: PSS-based nanocomposite (PEDOT: PSS/STP), towards Cu (II) ions from an aqueous medium.The factors affecting the adsorption of Cu (II) ion onto the nanocomposite PEDOT: PSS/STP including pH, temperature, contact time, and initial concentration were studied intensely.Isotherm, kinetics, thermodynamics, and reusability studies of the nanocomposite PEDOT: PSS/STP were also explored.

Synthesis of reagents
0.1 M solutions of both phosphoric acid and sodium tungstate were synthesised in double distilled water while the stannic chloride solution of 0.1 M was synthesised in 1 M hydrochloric acid solution.

Synthesis of poly (3,4-ethylene dioxythiophene) polystyrene sulphonate Tin (IV) tungstatophosphate nanocomposite (PEDOT: PSS/STP)
Using a simple bottom-up sol-gel approach, a PEDOT: PSS/STP nanocomposite cation exchanger was prepared in two steps.At room temperature, inorganic white STP precipitates were made by adding 0.1 M stannic chloride dropwise and stirring constantly to a mixture of 0.1 M sodium tungstate and 0.5 M orthophosphoric acid (H 3 PO 4 ) at a pH 0-1.This mixture was swirled continuously for 4 hours at 50°C on a magnetic stirrer.The CP\ PEDOT: PSS (2 ml) was added to the above-mentioned white precipitates in the second step, with continuous stirring on the magnetic stirrer for around 5 hours.The resulting pale blue precipitates were maintained for 24 hours to allow digestion.The light blue precipitates were filtered using a suction pump, washed with DDW, and dried in a hot air oven at 50° after the supernatant liquid was decanted off.The dry substance was broken down into small, homogeneous grains.After that, the ion exchanger was treated with 1 M nitric acid (HNO 3 ) for around 24 hours to convert it to H + .Finally, the nanocomposite (PEDOT: PSS/STP) was washed multiple times with DDW to remove any remaining acid and dried at 50°C in a hot air oven.By adjusting the quantity of PEDOT: PSS, several samples were obtained, and the IEC of each sample was calculated.Keeping in view IEC, the sample (S-5) was selected for more studies as shown in Electronic supplementary information ESI Table S1.

Characterisation
The characterisation of prepared samples was carried out by using an X-ray diffractometer (XRD) of Rigaku Smart Lab X-ray diffractometer, Fourier transform infrared (FTIR) spectrophotometer of Perkin Elmer Spectrum 2 MIR Spectrometer L1600235, transmission electron microscopy (TEM) of JEM2100, JEOL JAPAN, was utilised to provide the high-resolution images of the nanocomposite material and to investigate the particle size the material, field emission scanning electron microscope (FESEM) of Nova Nano FE-SEM 450, and energy-dispersive x-ray analysis (EDAX) were used to reveal the material's appearance and elemental composition, particle analyser (DLS) used for the measurement of surface charge of the material and thermogravimetric analysis (TGA) was performed by heating the prepared sample material to 900°C at a steady rate (10°C) in an air atmosphere.Mettler Toledo TGA/DSC 3+ GmbH, analytical, Germany, was used to perform the thermogravimetric analysis.During the synthesis of the samples, a hot air oven, muffle furnace, electronic balance, magnetic stirrer with a hot plate and a digital pH metre were also u.

Ion exchange capacity (IEC)
The IEC of PEDOT: PSS/STP was calculated using a conventional column technique as described in the literature [33,34].In this procedure, 1.0 gm of H + nanocomposite was placed in a glass column with an internal diameter of 1 cm and bottom support of glass wool.The eluent was a 1 M solution of metal nitrates of alkali metals.The eluent flow rate was maintained at 5 drops per mint.Using phenolphthalein as an indicator, the collected effluent was titrated against a reference solution of 0.1 M NaOH.For the calculation of IEC, the following formula was used: where M stands for molarity, V for alkali volume, and W for exchanger weight.
The sample S-5 shows a maximum IEC of 3.0 meq/g out of various samples prepared as shown above in ESI Table S1.

Experiment (batch adsorption)
The 1000 mg L −1 standard solution was made by dissolving a particular amount of anhydrous cupric chloride in demineralised water in this study.Following that, the standard solutions were diluted to the desired concentration.The starting pH of the solution should be about 7.0 to avoid the influence of insoluble Cu (II) precipitation on adsorption [35].Therefore, to assess the impact of initial pH on Cu (II) adsorption, the pH of Cu (II) solution (C 0 = 200 mg L −1 ) was adjusted between 2 and 11 using 0.1 M HCI or NaOH.
Adsorption steps: 50 mg PEDOT: PSS/STP was combined with 30 mL (1.66 g L −1 ) Cu (II) solution and swirled at 500 rpm for 40 minutes at 25°C with a constant starting pH.The samples were filtered through a syringe filter (0.45 m) once the adsorption was steady, and the Cu (II) concentration was estimated by atomic absorption spectrophotometry (AAS).Adsorption kinetics tests were performed at several intervals for 35 minutes to explore the Cu (II) adsorption mechanism and rate-controlling stages of PEDOT: PSS/STP.The diverse starting concentrations of Cu (II) (10-200 mg L −1 ) solution was used to generate the adsorption isotherms to examine the effect of different Cu (II) initial concentrations on the adsorption and interactions between adsorbent and adsorbate.For each set of adsorption studies, control samples were produced.

Results and discussion
Combination of conducting polymer with an inorganic material Sn (IV)tungstatophosphate can offer excellent capability in advanced functional materials.To develop conductive polymer-based nanocomposite having excellent heavy metal adsorption capability, the synthesis of PEDOT: PSS/STP nanocomposite was carried out via a bottomup sol-gel approach in an acidic medium.The overall synthetic procedure of PEDOT: PSS/ STP nanocomposite is shown in ESI Table S1.

FTIR (Fourier transform infrared) analysis
The FTIR spectra of the nanocomposite show a large peak at 3305-3311 cm −1 , which could be attributable to the presence of water molecules in addition to the -OH group.The bands at 2920 cm-1 and 2186 cm-1 are produced by aromatic C-H structure, whereas the bands at 1344 and 1645 cm-1 are produced by aromatic C = C and C-C stretching of aromatic rings (thiophene ring) [36].The peak at 1146 cm −1 is due to SO 3 − a group of PSS and peak at 1020 cm −1 is due to the ionic phosphate group (PO 4 3-) of STP, which confirms the formation of PEDOT: PSS/STP composite ion exchanger.Peaks at 766 and 851 cm −1 are attributable to the WO 4 −2 group, whereas peaks at 505 and 576 cm −1 are due to the Sn-O and O-Sn-O groups of STP.The FTIR spectrum's broadness is due to intramolecular or intermolecular hydrogen bonding between the inorganic part and the organic polymer [37][38][39].These results indicate the formation of nanocomposite material by binding inorganic material with the organic polymer (Electronic supplementary information ESI Figure S1).
The FTIR spectra of PEDOT: PSS/STP before and after adsorption in Figures S1 and S2 shows that the stretching vibration of the OH band is broadened, and change in stretching frequencies of Sn-O, WO 4 2-and PO 4 3-from 505 cm −1 , 766 cm 1-and 1020 cm 1-to 497 cm −1 , 777 cm 1-and 1027 cm 1-, this shows that these groups have a significant electrostatic attraction with the Cu (II) ions.Also, there appears a peak at 383 cm −1 which may be due to the presence of Cu 2+ in the material, and the absorption band at 1146 cm −1 due to the SO 3 − group disappears as shown in ESI Figure S2, which might be due to the adsorption of Cu (II) ion.

Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG)
The bonding strength of intercalated water molecules absorbed physically or chemically has a substantial influence on their thermal breakdown.A TGA/DTG analysis was used to explore the thermal behaviour of the produced products.ESI Figure S3(a,b) illustrate the results of the TGA/DTG analyses of the STP and PEDOT: PSS-STP respectively.The TGA graph or thermogram of the material in H + form shows a continuous weight loss of 12% when heated up to 100-150°C which is due to the evacuation of water molecules presenting the nanocomposite [36].Then, about 8% weight loss occurs from 150°C to 400°C which is due to the formation of pyrophosphate from the inorganic phosphate group and deuteriation of organic polymer PEDOT: PSS [36].Gradual weight loss of about 5% takes place from 400°C to 600°C which is due to the evacuation of internal water molecules and deuteriation of organic polymer PEDOT: PSS [36,40].Beyond 600°C, the development of mixed oxides of Stannic, tungsten, and phosphate may account for the additional weight loss of roughly 5% [36].The DTG curve displays four distinct peaks at 150°C, 300°C, 400°C, and 600°C, confirming the TGA analysis transitions.

Powder X-Ray diffractometry (XRD) analysis
STP and PEDOT: PSS/STP (S-5) revealed very minor peaks of 2ɵ values in the X-ray powder diffraction pattern, indicating that the nanocomposite cation exchanger is amorphous.The inorganic cation exchanger (STP) shows a broad peak at around 51.40° 2ɵ value which also appears in composite exchanger PEDOT: PSS/STP at around 55.3° 2ɵ value [41].However, in composite cation exchanger, there appear two additional peaks at 8.24° and 27.94° 2ɵ values.The peak at 8.24° is due to the PSS part of the conducting polymer and the peak at 27.94° is because of the PEDOT part of the polymer [42][43][44].From the above discussion, it is clear that the nanocomposite cation exchanger (PEDOT: PSS/STP) has been synthesised successfully (ESI Figure S4).

FESEM (Field emission scanning electron microscopy) and EDAX (energy dispersive x-ray analysis)
FESEM images of STP and PEDOT: PSS/STP nanocomposite at different magnifications are shown in ESI Figure S5.The images Figure S5(a,b) confirmed that STP nanoparticles are irregular flake-shaped.The surface morphology was entirely transformed into a rough, granular, and uneven surface, indicating the production of the nanocomposite.When considering nanocomposite cation exchanger (PEDOT: PSS/STP), inorganic cation exchanger (STP) nanoparticles are non-uniformly dispersed onto the surface of conducting polymer (CP).The nanoparticles of STP are visible on the surface of CP, as seen in the inset in ESI Figure S5(c,d).Furthermore, when examining STP nanoparticles, substantial clumping of single nanoparticles may be observed and corroborated by other researchers [45,46], however, no clumping of free nanoparticles can be detected in the PEDOT: PSS/STP nanocomposite.We were able to confirm that nearly all of the accessible STP nanoparticles were accumulated on the PEDOT: PSS surface as a result of our FESEM analysis.
The numerous elemental peaks of Sn, W, P, O, S and C found in nanocomposite PEDOT: PSS/STP was confirmed by EDAX, confirming the development of an ion exchanger by combining the inorganic portion (STP) and the organic counterpart (PEDOT: PSS) as shown in ESI Figure S5(e).The histogram shows the average particle size of the nanocomposite material drawn from the FESEM Figure S5(f).

Transmission electron microscopic (TEM) Analysis:
ESI Figure S6(a,b) shows typical TEM images of PEDOT: PSS/STP nanocomposite at different magnifications.The structure of PEDOT: PSS/STP nanocomposite constituted of small amorphous nanoparticles can be observed in the TEM pictures of the nanocomposite ion exchanger (PEDOT: PSS/STP) flake ESI Figure S6(a,b).The existence of a predominantly amorphous phase of PEDOT: PSS/STP is also indicated by electronic diffraction collected from the surface of nanoparticles ESI Figure S6(c).From the TEM images, it is confirmed that every particle indicates its different structure and composition due to their different contrast.The darker portion in the TEM images shows the organic part wrapped with inorganic part i.e.STP, and the greyish part indicated the presence of organic part i.e.PEDOT: PSS.It also shows the presence of STP bright particles with a nonhomogeneous distribution.ESI Figure S6(d The surfaces of both STP and PEDOT: PSS/STP ion exchangers are negatively charged at pH greater than its IEP, which is helpful in the adsorption of cations from the environment [47].The ζ is primarily determined by two factors: pH and material concentration [48].In this investigation, we set the sample concentration at 0.02-0.03g in 10 ml DDW and studied the ζ as a function of pH. The introduction of the PEDOT: PSS enhances the ζ of the inorganic counterpart, as evidenced by the ζ measurements.The ζ of the polymer PEDOT: PSS is around −80 mV [49], which is quite high when compared to the inorganic counter-part STP (−26.125 mV) and the nanocomposite PEDOT: PSS/STP (−38.95 mv).As a result of the PEDOT: PSS being incorporated into the inorganic counter-part STP, the ζ and thus the composite's stability in aquatic environments has been increased.
After adsorption of Cu (II) ions from its solution, the ζ of the nanocomposite material gets less and less negative as shown in ESI Figure S7, which confirms the adsorption of Cu (II) ions by either electrostatic interaction or ion exchange mechanism.

Ion exchange capacity (IEC) analysis
Ion exchange is a process in which ions attached to a high molecular weight polymer are exchanged for other ions in solution.The high molecular weight polymer is normally in the form of a small round sphere called beads.Ion exchange resins are used to demineralise water and to separate mixtures of ions.One important characteristic of ion exchange resin is its capacity, expressed in terms of milliequivalents of exchangeable ions per gram of resin.Exchange capacity can be measured by exchanging sodium ions (Na + ) for hydrogen form (H + initially attached to resin), the hydrogen ion is then titrated with Sodium hydroxide solution.The exchange reaction can be written as: In the equation, R represents the ion resin.
Standard column technique was used to measure the IEC of the inorganic part (STP) and nanocomposite (PEDOT: PSS/STP).In comparison to the inorganic part (1.25 meq/g), the IEC of the nanocomposite (3.0meq/g) is outstanding, which is due to the sulphate (SO 3 − ), hydroxide (OH − ), and PO 4 3-ions present on the nanocomposite [50,51].
In the current study, the IEC of the hybrid nanocomposite with alkali metal ions increases as the hydrated ionic radii of alkali metal ions decrease.Ions with smaller hydrated ionic radii penetrate the ion exchanger pores more easily, resulting in a higher IEC.This observation is consistent with the findings of other researchers [52,53].ESI Table S2 demonstrates the IEC of nanocomposite material with different ions (K + >Na + >Li + ).

Surface charge analysis by zero point charge
The pH at which the overall charge on the adsorbent surface is zero is defined as the zeropoint charge (ZPC).The adsorbent surface becomes positively charged below pH ZPC , whereas it becomes negatively charged beyond pH ZPC .However, the mechanism of adsorption is not just dependent on pH ZPC ; other components also influence the adsorption of substances onto the adsorbent.The salt addition technique [54] was used to evaluate the pH ZPC of PEDOT: PSS/STP and STP.A sequence of 10 mL volumetric flasks containing 0.5 mg of PEDOT: PSS/STP and STP were with 50 mL of 0.1 M Potassium nitrate and the initial pH was altered from 1 to 12 using 0.1 M HCl and 0.1 M NaOH.The flasks were maintained in a water bath at a fixed temperature for 24 hours.A pH metre was used to determine the ultimate pH of the supernatant solution.The ionisation of different contaminants, as well as the surface of the adsorbent, is influenced by the pH of the solutions.The pH ZPC was obtained to be 7.4 and 7.75, respectively, from the plot of ΔpH (pH i -pH f ) versus initial pH i (ESI Figure S8) [55].

Dependance on solution pH
The pH of the solution is a key aspect to take into account throughout the adsorption process, because adsorbent surface charges and diversification can fluctuate under different pH circumstances, influencing the adsorption behaviour of the adsorbent [56,57].For instance, Cu (II) appears in several forms such as Cu (OH) 2 , Cu (OH) + , and Cu 2+ , at distinct pH values [5,9].Interestingly, the zeta potential of PEDOT: PSS/STP before Cu (II) adsorption dropped (-ve values) as the pH increased (ESI Figure S7), and PEDOT: PSS/STP exhibited a negative charge on the surface as the sequence progressed.Electrostatic interactions could be implicated in the Cu (II) adsorption process, according to the negatively charged surface of PEDOT: PSS/STP.The adsorption of Cu (II) by PEDOT: PSS/STP was explored to discover how pH affected the adsorption of Cu (II).Cu (II) adsorption capabilities onto PEDOT: PSS/STP enhanced with initial pH values ranging from 2.0 to 7.0, as illustrated in ESI Figure S9, and decreased beyond pH 7. Because protons in solution compete with heavy metal ions at low pH values, whereas most heavy metals precipitate at high pH, therefore strongly alkaline or acidic conditions are thought to be inappropriate for adsorption [57], as shown in Figure S8, the PEDOT: PSS/STP had the most negatively charged surface and the largest adsorption capacity within the observed pH (85.05 mg g −1 ).The surface charge of nanocomposite material PEDOT: PSS/STP was also monitored during (before and after) adsorption to investigate the role of electrostatic or ionic interactions in the adsorption process (ESI Figure S7).The Cu (II) addition altered the zeta potential in the PEDOT: PSS/STP material towards the positive directions (i.e. less negative).When it came to Cu (II) adsorption at various pH levels, the PEDOT: PSS/STP showed the maximum Cu (II) adsorption capacity at pH 7.0.

Dependance on contact time
ESI Figure S10 depicts the fluctuation in adsorption as the contact time varies.The main measure utilised to explore the kinetics of the adsorption procedure is contact time [5].All of the sites on the adsorbent are initially unoccupied, therefore interactions result in enhancement in the absorption of Cu (II) ions as contact time between the adsorbate and the adsorbent increases until the adsorption equilibrium is reached at 35 minutes.The quantity of Cu (II) ions adsorbed equals the quantity of Cu (II) ions desorbed after 35 minutes, resulting in no net increase in adsorption.

Dependence on initial concentration
The starting concentration of adsorbate is crucial while investigating the kinetics of adsorption [58].Various concentrations of Cu (II) ion, ranging from 10 to 200 mg/L, were used to investigate the influence of the adsorbate's starting concentration.It was discovered that as the concentration of Cu (II) ions was increased from 10 to 200 mg/L (ESI Figure S11), the adsorption of Cu (II) ions using PEDOT: PSS/STP nanocomposite decreased, which could be due to the lower availability of freely accessible groups at the surface of PEDOT: PSS/STP nanocomposite for the higher concentration of Cu (II) ions.

Dependance on temperature
In adsorption reactions, the temperature is a key variable.Other parameters were kept constant (Co -200 mg/L; pH-7; and contact time -35 min) while the temperature was changed between 298 and 312 K.As shown in ESI Figure S12, the removal % of Cu (II) utilising PEDOT: PSS/STP nanocomposite substantially increased as the temperature raised from 298 to 312 K, then remained constant.The fact that adsorption enhanced with temperature suggested a significant adsorption relationship between the PEDOT: PSS/STP nanocomposite surface and Cu (II) ions, which facilitated chemisorption.The findings were consistent with those of prior investigations by other researchers [5,58].

Adsorption kinetics
The purpose of the kinetics investigation was to determine the rate of Cu (II) uptake on the PEDOT: PSS/STP adsorbent.ESI Figure S12 depicts the outcomes of the experiment of Cu (II) adsorption on PEDOT: PSS/STP vs time at 30°C.According to ESI Figure S13 the adsorption equilibrium for PEDOT: PSS/STP was reached in 35 minutes, which was acceptable in practice.The models like pseudo-first-order and pseudo-second-order were utilised to explore the kinetics data to learn more about the adsorption mechanism.These models are generally described as Equations ( 2) and (3), accordingly [59,60]: The quantity of Cu (II) adsorbed (mg g −1 ) at equilibrium and at time t (min), accordingly, is q e and q t , and pseudo-first-order rate constant is k 1 (min −1 ) and pseudo-second-order rate constant is k 2 (g mg −1 min −1 ).
The aforementioned kinetics models have been utilised to match the experimental data, and the results obtained were displayed in ESI Table S3.The pseudo-second-order model was found to be more adequate to represent the kinetics behaviour for Cu (II) ions onto the PEDOT: PSS/STP relying on the examination of the R 2 of the linear form for various kinetics models (Table S3).Furthermore, the matching results of the pseudosecond-order model were shown in ESI Figure S13.The experimental data closely resembles the theoretically anticipated curve as fine solid lines, demonstrating a pseudo-second -order kinetic process of adsorption onto PEDOT: PSS/STP, with chemisorption as the ratecontrolling mechanism.They were completely consistent with the results of the adsorption isotherm study, which are discussed further below.Furthermore, as shown in ESI Table S3, the correlation coefficient of the pseudo-first-order model was also greater than 0.9, signifying that the adsorption mechanism based on this model may be somewhat involved in the adsorption process alongside the pseudo-second-order kinetics.

Adsorption isotherms
The isotherm is essential for describing the interactions between solutes and adsorbents, as well as elucidating the adsorbent's characteristics and reactivity [61].The Cu (II) adsorption isotherms of PEDOT: PSS/STP were measured at pH 7.0 in this study, and the results are shown in ESI Figures S14-S17.The Cu (II) uptakes of PEDOT: PSS/STP grew linearly with increasing Cu (II) concentrations at first, then approached surface saturation at high concentrations, according to these figures.It was discovered that the adsorption sites on the adsorbents were sufficient at lower beginning Cu (II) ion concentrations, and Cu (II) uptakes were dependent on the quantity of Cu (II) ions transported from the bulk of the solution to the adsorbents' surface.The adsorption sites on the surface of the adsorbent attained saturation at increasing starting concentrations of Cu (II) ions, and the adsorption of Cu (II) ions reached equilibrium [62].To further study the adsorption mechanism, the adsorption data were evaluated using several isotherm models such as Freundlich, Langmuir, Dubinin-Radushkevich (D-R) and Temkin isotherm models.Only monolayer adsorption happens in Langmuir's model, which is predicated on the presumption that adsorption sites are equal and energetically similar [61].It can be described as follows: where Qe is the quantity of Cu (II) ions adsorbed at equilibrium (mg g −1 ), Q max is the adsorbent's maximum adsorption capacity (mg g −1 ), Ce is the liquid-phase Cu (II) concentration at equilibrium (mg dm −3 ), K L is the Langmuir adsorption constant (dm −3 mg −1 ), and the Freundlich model assumes an adsorption site energy distribution that decays exponentially [63].It is used to define a diversified system with an n-fold heterogeneity factor.This is how the Freundlich model is represented: The Freundlich constant is K f , and the diversity factor is n (dimensionless).The Temkin adsorption isotherm assumes that the adsorbents' surface is uneven and that the energy of the binding sites is redistributed linearly.It's utilised to investigate the chemical adsorptions [64].
B T is a constant associated with adsorbent surface diversity (mg g −1 ), and A T is a constant associated with initial adsorption energy (mg g −1 ).The linear form of Dubinin-Radushkevich (D-R) isotherm is as [55]: Q m (in mgg −1 ) denotes D-R adsorption capacity; K DR represent D-R isotherm constant and ε specifies the Polanyi potential.ESI Table S4 lists the constants as well as the correlation coefficients.The Langmuir model fit the data better than the Freundlich model, indicating that the Cu (II) adsorption process via PEDOT: PSS/STP was primarily a single layer.With increasing temperature, Q max (80.84-85.04)increased, indicating that PEDOT: PSS/STP produced at high temperature had a larger adsorption impact.The Temkin model's results likewise revealed a high degree of fitting, indicating that PEDOT: PSS/STP possessed strong intermolecular forces during the Cu (II) adsorption process.Overall, a temperature rise may increase the capability of Cu (II) adsorption.
A dimensionless constant, R L , can be derived from the constant K L for further investigation of the adsorption process, reflecting the basic characteristic of the Langmuir model [59]: where C 0 is the Cu (II) ion concentration in the solution at the start.The R L number indicates whether the adsorption process is undesirable (R L > 1), linear (R L = 1), desirable (0 < R L < 1), or irreversible (R L = 0) [59].When R L is smaller (less than one), it is assumed that there is a stronger affinity between the adsorbate and the adsorbent.ESI Table S4 shows that the values obtained for the adsorbent at various temperatures were less than 1.0, showing that Cu (II) ions adsorb effectively.
The adsorption favourability of n −1 is indicated in the Freundlich model.If n −1 is less than 1.0, the adsorption strength is good (or beneficial) across the entire concentration range examined; whereas, if n −1 is greater than 1.0, the adsorption strength is good (or beneficial) at high concentrations but not so much at lower concentrations [59].For PEDOT: PSS/STP, the value of n −1 is smaller than 1.0, showing strong adsorption over the whole range of concentrations.The constant A T in the Temkin model, on the other hand, represents the starting adsorption energy for an adsorbent; a higher A T value represents stronger adsorption energy as well as a larger selectivity for the adsorbent surface.
Adsorption energy (E) is one of the variables used in the D-R isotherm model to assess the nature of adsorption.It is calculated using the following formula: The D-R isotherm constant is denoted by K DR .The value E denotes whether the phenomenon is physical or chemical adsorption.Adsorption energies for Cu (II) ions were found to be greater than 100 KJ/mol in this investigation at different temperatures as shown in ESI Table S5.It was chemisorption in nature, according to the results [55].ESI Table S5 lists the various parameters of D-R model.

Adsorption thermodynamics
The thermal efficiency of adsorption can be applied to forecast the feasibility study and course of adsorption processes, as shown in ESI Figure S18.Thermodynamic characteristics such as a change in Gibbs free energy (ΔG o ), change in entropy (ΔS o ), and change in enthalpy (ΔH o ) can be employed to understand and forecast the course of adsorption.To derive these thermal characteristics, apply the following Equations (8 & 9) [60,65]: K c stands for Langmuir constant, R stands for universal gas constant (8.314JK1), and T stands for absolute temperature in Kelvin (K).The slope and intercept of the plot of ln Kc versus 1/T, accordingly, can be used to calculate the values of ΔH o and ΔS o .
ESI Table S6 displays the result of all of these thermodynamic characteristics.ΔH° and ΔS° were estimated to be 89.84(J mol −1 ) and 222.81 (J mol −1 K −1 ), respectively.ΔG° was determined to be 8.0, 7.5, and 7.2 k j mol −1 at 298, 303, and 312 K, respectively.Cu (II) adsorption onto the PEDOT: PSS/STP nanocomposite was particularly suitable at elevated temperatures, as seen by the drop in ΔG° values when the temperature was raised [66].The endothermic character of Cu (II) adsorption onto PEDOT: PSS/STP nanocomposite was revealed by the positive value of ΔH o (84.59 kJ/mol).The fundamental value of ΔH o tells you a lot about the type of adsorption process you're dealing with.The fundamental values of ΔH o for adsorptions (physical and chemical) range from 2.1 to 20.9 kJ/mol and 20.9 to 418.4 kJ/mol, accordingly [67].Cu (II) adsorption onto PEDOT: PSS/STP nanocomposite was determined to be 89.84kJ mol −1 , indicating that the adsorption was chemisorption, as shown in Table S5.Furthermore, the enhanced randomness at the phase boundaries during Cu (II) adsorption onto PEDOT: PSS/STP nanomaterials was revealed by the +ve values of ΔS°.

Mechanism of adsorption
Metal ion adsorption on the adsorbent can occur through a variety of interactions at the adsorbent's interface, including the coordination of metal ions by functional groups (OH), ion exchange, or electrostatic interactions.So, the removal of copper ions may be either by electrostatic interactions (with OH − , SO  , SO 3 − , and PO 4 3-, which indicates the strong electrostatic (i.e.ionic) interaction of these groups with the Cu (II) ions, also there appears a peak at around 383 cm −1 which may be due to the Cu 2+ present in the composite (ESI Figure S4).Furthermore, as the investigated pH increased, the zeta potential of PEDOT: PSS/STP before Cu (II) adsorption dropped, and PEDOT: PSS/STP had negative surface charges.Ionic interactions could be implicated in the Cu (II) adsorption process, according to the negatively charged surface of PEDOT: PSS/STP.The Cu (II) adsorption of PEDOT: PSS/STP was studied to see how pH affected the adsorption of Cu (II).With starting pH values ranging from 2.0 to 11.0, the adsorption capabilities of Cu (II) onto PEDOT: PSS/STP increased, as shown in ESI Figure S7.Strong conditions i.e. acidic and alkaline are thought to be undesirable for adsorption because protons compete with heavy metal ions at low pH values, whereas precipitation of most heavy metals takes place at high pH levels (i.e.alkaline conditions) in the solution [57].In specific, the Cu (II) addition moved the values of zeta potential towards positive or less negative directions in the PEDOT: PSS/STP system.For the PEDOT: PSS/STP, the maximum Cu (II) adsorption capacity was recorded at pH 7.0.

Reusability studies
To enhance the cost-effectiveness of the process, the metal ions must be desorbed and the materials must be reused for additional cycles.Cu (II) desorption from the PEDOT: PSS/STP nanocomposite surface was achieved using a 0.1 M HCl solution.The introduction of protons, which competed with Cu (II) ions for binding sites, caused Cu (II) desorption.Cu (II) ions on the nanocomposite surface were replaced by protons in solution in 0.1 M HCL solution.90% Cu (II) was adsorbed in the first cycle, with 94% desorption was obtained.67% elimination and 70% desorption were accomplished even in the fifth cycle (ESI Figure S19).As a result, only those Cu (II) ions that were attached to the nanocomposite surface by physical forces were desorbed during the desorption process.Between the nanocomposite and Cu (II) ions, it was likely that both ion exchange and electrostatic interactions occurred.As a result, there was no way to achieve total desorption [58].According to the recyclability investigations, the adsorbent can be employed as an efficient adsorbent in wastewater treatment innumerable times without losing much of its adsorption ability.

Conclusion
Potentially toxic metal ions from a wide range of sources enter water systems, affecting various aquatic ecosystems.Conducting polymer-based nanocomposite has grown in importance as effective upcoming material, attracting considerable interest regarding the potential in the elimination of pollutants from water bodies.PEDOT: PSS/STP nanocomposite was used as an adaptable adsorbent to remove hazardous Cu (II) ions from an aqueous solution in this study.The PEDOT: PSS/STP nanocomposite exhibited rapid Cu (II) adsorption behaviour in 35 minutes, outstanding monolayer adsorption capacities (85.05 mg/g) at 312 K, and improved reusability, with almost 70% of Cu (II) absorption persisting after five regeneration cycles.The pseudo-second-order kinetics and Langmuir model fitted the adsorption kinetics and isotherms better, accordingly.In summary, the PEDOT: PSS/STP nanocomposite exhibited remarkable Cu (II) ion elimination and is predicted to be used in the treatment of wastewater.

3 . 1 . 5 .
), is a Histogram image and shows the average particle size(13.4nm) of the nanoparticles of the cation exchanger PEDOT: PSS/STP Zeta potential (ζ) analysis PEDOT: PSS/STP nanocomposite flakes and STP flakes have their zeta potentials (ζ) tested.The ζ measurements were done independently for each of the experimental setups, which included PEDOT: PSS/STP nanocomposite flakes suspensions and STP flakes suspensions in DDW.The ζ values were calculated as a function of pH.To automatically modify the pH readings, 0.1 M solutions of HCl and NaOH were employed as titration media.The findings of the ζ studies for the investigated suspensions are shown in ESI Figure S7.They show that the ζ of the nanomaterials studied was pH-dependent in the majority of cases.The ζ readings obtained for STP and PEDOT: PSS flakes suspended in DDW in an acidic environment (pH 2) were marginally positive.The ζ of PEDOT: PSS/STP nanocomposite flakes is −38.95 mV at a pH of 7.0 however, at the same pH the ζ of inorganic flakes (STP) was only −26.125 mV which explains the stability of the nanocomposite hybrid materials in the water as compared to the inorganic material.We also measured the Zeta potential of the PEDOT: PSS/STP nanocomposite material and STP at different pH values ranging from 2 to 11, and found that as the pH rises, the negative value of the ζ rises with it, with the highest ζ values of −38.95 mV and −26.125 mV, respectively, at pH 7.0 and beyond the pH 7.0 the negative ζ values decrease.The ionisation of sulphate (SO 3 − ), phosphate (PO 4 3-), and hydroxyl (OH − ) groups present on the surface of PEDOT: PSS/STP and STP ion exchanger materials cause an increase in the negative value of ζ.
exchange mechanism in which Cu2+ or Cu (OH) + can be exchanged with WO 4 2-and OH − ions.This was also confirmed by the observed shift in the peaks corresponding to WO .FTIR (ESI Figures S1 and S2 ) and zeta potential (ESI FigureS7) measurements were used to appropriately explain the adsorption of Cu (II) ions onto PEDOT: PSS/STP nanocomposite in the present study.From the FTIR spectrum of the PEDOT: PSS/STP during (i.e.before and after) adsorption in ESI FigureS1and ESI FigureS2, it is evident that there occurs broadness in the stretching vibration of −OH band, and a slight change in stretching frequencies of WO 4 2-