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Heavy metal ions are highly toxic to health, lung, kidney and nervous systems. A special featured polypyrrole with environmental stability and reusability was engaged for removal of metal ions from aqueous solution. In the present study, a new nanocomposite of polypyrrole (PPy/Ag2O) in the presence of hydroxypropylcellulose (HPC) as the surfactant was synthesized via in situ chemical oxidative polymerization method in aqueous medium. The synthesized nanocomposites were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Results indicated that the Ag2O and hydroxypropylcellulose (HPC) influenced the properties of the synthesized nanocomposites. The aim of this research was to investigate the sorption characteristics of PPy/Ag2O nanocomposites for the removal of heavy metal ions including Pb(II) and Co(II) from aqueous solution. The factors that affected the adsorption equilibrium as well as the removal efficiency of the nanoadsorbents, i.e., contact time, metal ion concentration, pH and adsorption conditions, were investigated in detail. The equilibrium data were fitted to Langmuir, Temkin and Freundlich isotherm models. From the kinetic results, it was concluded that the pseudo-second-order kinetic model was the best at describing the adsorption process for Pb(II) and Co(II) on PPy-HPC/Ag2O.

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  • A new nanocomposite containing Ag2O was synthesized for the removal of heavy metal ions.

  • The synthesized nanocomposite was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy and XRD.

  • Sorption mechanism, isotherm, kinetics and thermodynamic studies were carried out.

  • The factors that affected the adsorption equilibrium and the removal efficiency of the nanoadsorbents, i.e., contact time, metal ion concentration, pH and adsorption conditions, were investigated in detail.

  • Desorption and regeneration studies were also performed.

adsorption, kinetic, nanocomposite, polypyrrole, thermodynamic
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The pollution of heavy metals from industrial wastewater has become a global issue. Heavy metals are metallic chemical elements that have a high density and are toxic at low concentrations. With the development of industries such as metal finishing, ceramics, pulp, lead smelters, electroplating, mining, batteries, fertilizers and paper industries, heavy metal ions are discharged into rivers, lakes and ocean environments (Guo et al. 2022; Ghasemi et al. 2023). Heavy metal exposure causes serious health effects, such as cancer, organ damage, nervous system damage, lung damage and damage to the brain, and eventually leads to death. Therefore, it is necessary to remove the heavy metal ions before they are released into the environment (Ghamari et al. 2022).

Many techniques, such as coagulation–flocculation (Montaño-Medina et al. 2023), ion exchange (Tavakoli et al. 2017), physical and chemical precipitation (Türk et al. 2022), membrane filtration (Li et al. 2023), electrochemical treatment (Fang et al. 2023), reverse osmosis (Khan et al. 2023) and adsorption processes (Jing et al. 2023), are extensively used methods for removing heavy metal ions from an aqueous medium. Among the technologies, adsorption technology is the best technology. Adsorption is a low-cost industrial separation technique, easy to operate and particularly effective. In addition, it does not result in the formation of harmful substances (Zavala & Bouchez 2022; Arenas et al. 2023; Pan et al. 2023).

Finding new materials for adsorbents with high adsorption performance and selectivity remains challenging. Different types of adsorbents have been proposed including active carbon (Joshi et al. 2022), ferromagnetic materials (Song et al. 2022), polymer (Karimi et al. 2021) and biosorbents (Dey et al. 2022). Polymers, which are characterized by reactive functional groups containing O, N, S and P atoms, have gained great attention as an effective adsorbent for heavy metals due to the high ability and affinity of both nitrogen and sulfur atoms to bind with the metal ion (Khalili et al. 2021; Stejskal et al. 2023). Conducting polymers, such as polyaniline, polypyrrole (PPy) and polythiophene with interesting doping capabilities, have been identified as candidates for adsorbents (Kharazi et al. 2018; Morsi et al. 2018; Jurča et al. 2022). As one of the novel conducting polymers, polypyrrole (PPy) is a conjugated polymer with alternating single and double bonds (Barkade et al. 2023). In order to obtain an adsorbent with high metal ion sorption efficiency, this polymer can be used combined with various materials such as multiwalled carbon nanotubes (MWCNTs) (Yan et al. 2022) and metal oxides (Sarojini et al. 2021; Amara et al. 2023). These composites have the properties of metals and polymers and also exhibit many new characteristics that single-phase materials do not have (Hosseini et al. 2017). There are various methods for the synthesis of conductive polymers, among which the in situ polymerization method is of particular interest. The advantages of this method include the ability to produce polymer products with required properties and characteristics by adjusting process conditions, reducing waste and production costs, reducing production time and reducing the need for post-processing steps (Bayramoglu et al. 2024; Bayramoglu & Yakup Arica 2024).

Hence, in this study, polypyrrole containing nanometre-sized Ag2O was synthesized via the in situ chemical oxidative polymerization method in the presence of hydroxypropyl cellulose (HPC) as a surfactant. The structure and morphology were analyzed by Fourier transform infrared (FTIR), X-ray diffraction and scanning electron microscopy (SEM). Using these nanocomposites as absorbents, the removal of heavy metal ions, such as Pb(II) and Co(II), from water was investigated. The influences of various parameters, such as contact time, initial concentration of heavy metal ions and pH on the sorption capacity, were investigated. Also, the adsorption kinetics, equilibrium and thermodynamics were discussed.

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Instrumentation

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A magnetic mixer model MK20 (Germany), pH meter model HANNA211, centrifuge model Z-36HK, FTIR spectrometer Thermo Nicolet model Nexuf 670, XRD model Equinox 3000, SEM model KYKY-EM3200, flame atomic absorption spectrophotometer model Thermo electron, oven Binder model FD 23 and digital scale model FR 200 were employed.

Materials

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All reagents were used as received without further purification. Materials used in this work were monomer of pyrrole (d = 0.97 g/cm3), nanometre-sized silver oxide (Ag2O) from Aldrich, ferric chloride (FeCl3) and HPC obtained from Merck (Schuchardt, Germany). Pyrrole monomer was purified by simple distillation before use. Distilled water was employed throughout this work.

Preparation of polypyrrole/Ag2O in the presence of HPC as the surfactant

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The polypyrrole was synthesized chemically using FeCl3 as an oxidant in aqueous media. For the synthesis, 5 g of FeCl3 was added in 50 mL of distilled water. Then 0.2 g of HPC and 1.0 g of Ag2O were dissolved in 50 mL of distilled water, a uniform solution was achieved using a magnetic mixer for 20 min and added to the oxidant solution. Finally, 1 mL of pyrrole monomer was added to a stirred aqueous solution, which was maintained under constant stirring. The reaction was carried out for 5 h at room temperature. The product was collected by filtration. In order to separate the oligomers and impurities, the product was washed several times with deionized water. It was then dried at about 60 °C in an oven for 24 h. As a reference sample, a pure PPy was synthesized using the same method used previously, without Ag2O and HPC (i.e., 100 mL of distilled water containing 5 g of FeCl3 and 1 mL of pyrrole monomer).

Batch adsorption experiment

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The application of PPy-HPC/Ag2O for the removal of Pb(II) and Co(II) ions from the aqueous solution was investigated using the batch equilibrium method. The standard heavy metal solutions used in the study were prepared from Pb(NO3)2 and CoSO4·7H2O, respectively, with deionized water. All samples were shaken in 30 mL of heavy metal solution at an agitation speed of 400 rpm. The pH values of the solution in these experiments were approximately 2–7. For determining the amount of the removal of metal ions by nanocomposites, the metal ion solution (50 mg/L) was mixed with 0.3 g of powder adsorbents and then stirred using a magnetic mixer for 45 min. At the end of predetermined time-intervals, the sorbate was filtered and the concentrations of Pb(II) and Co(II) were determined by atomic adsorption. All experiments were carried out twice to obtain average values. The deviation of this method was found to be less than 3%. The pH of the solution was adjusted using 0.1 mol/L NaOH and/or 0.1 mol/L HNO3, and the pH measurements were carried out in a digital pH meter. The removal efficiency of Pb(II) and Co(II) and the amount of metal ions adsorbed per specific amount of adsorbent at equilibrium (mg/g), qe, were calculated as follows:
(1)
(2)
where qe is the amount of metal adsorbed per specific amount of adsorbent (mg/g), C0 and Ce are the initial and equilibrium solution concentrations (mg/L), respectively, V is the volume of solution (L) and m (g) is the weight of adsorbent.
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Characterization of nanocomposites

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The FTIR spectroscopy of polypyrrole composites is shown in Figure 1. The FTIR spectroscopy provided valuable information regarding the formation of nanocomposites. Figure 1 shows the FTIR spectra of (a) pure PPy, (b) PPy-HPC, (c) PPy/Ag2O and (d) PPy-HPC/Ag2O. The band at 1,542 cm−1 corresponds to the C–C and C = C stretching vibrations and that at 1,299 cm−1 reflects the C–N stretching vibration. The band from 1,400 to 1,250 cm−1 is attributed to C–N in-plane deformation modes and has a maximum of 1,299 cm−1. The bands of the C–H and N–H in-plane deformation vibrations are situated at 1,045 cm−1. The absorptions at 790 cm−1 are related to the C–H outer bending vibrations (Kalombo et al. 2021). The above results indicate the formation of PPy in the composite. As shown in Figure 1(b)–1(d), the existence of HPC and Ag2O imposed the same effect as the one on pure polypyrrole, which results in the observed wavenumber shift and peak intensity change.
Figure 1
FTIR spectra of (a) pure PPy, (b) PPy-HPC, (c) PPy/Ag2O and (d) PPy-HPC/Ag2O nanocomposites.
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FTIR spectra of (a) pure PPy, (b) PPy-HPC, (c) PPy/Ag2O and (d) PPy-HPC/Ag2O nanocomposites.

Figure 1
FTIR spectra of (a) pure PPy, (b) PPy-HPC, (c) PPy/Ag2O and (d) PPy-HPC/Ag2O nanocomposites.
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FTIR spectra of (a) pure PPy, (b) PPy-HPC, (c) PPy/Ag2O and (d) PPy-HPC/Ag2O nanocomposites.

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The crystalline nature of nanocomposites was determined from the XRD analysis. The XRD patterns of (a) pure PPy, (b) Ag2O and (c) PPy/Ag2O are shown in Figure 2, respectively. The XRD pattern of pure PPy (Figure 2(a)) shows a significant peak at 2θ = 26.13°, which represents the semi-amorphous structure of polypyrrole. The XRD pattern of Ag2O is shown in Figure 2(b). The main peaks in Figure 2(b) are at 2θ = 27.21°, 2θ = 32.57°, 2θ = 49.72° and 2θ = 59.36°. The mentioned peaks can be seen in the XRD pattern of PPy/Ag2O, which confirmed the existence of Ag2O nanoparticles in the polypyrrole matrix. The crystallinity of PPy has been enhanced after grafting with silver oxide, and this may be due to the intermolecular interaction between the silver oxide and PPy.
Figure 2
XRD patterns of (a) pure PPy, (b) Ag2O and (c) PPy/Ag2O nanocomposites.
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XRD patterns of (a) pure PPy, (b) Ag2O and (c) PPy/Ag2O nanocomposites.

Figure 2
XRD patterns of (a) pure PPy, (b) Ag2O and (c) PPy/Ag2O nanocomposites.
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XRD patterns of (a) pure PPy, (b) Ag2O and (c) PPy/Ag2O nanocomposites.

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The morphological characterization of products was analyzed using a scanning electron microscope.

Figure 3 shows the SEM images obtained for (a) pure PPy, (b) PPy-HPC, (C) PPy/Ag2O and (d) PPy-HPC/Ag2O. Polypyrrole composites have spherical and porous structures in aqueous media (Tan & Wang 2021). Figure 3(b) shows the PPy-HPC nanocomposite. As can be seen, the surface of HPC is covered with uniformly dispersed PPy particles. Surfactants influence the size distribution, morphology and homogeneity of particles because they prevent the gross aggregation of the particles (Ibrahim et al. 2022). Figure 3(c) and 3(d) show the PPy–Ag2O and PPy-HPC/Ag2O nanocomposites, respectively. As can be observed in the micrographs, the particle size decreased and the homogeneity of particles increased. The particles of Ag2O are coated not only on the surface of the particles but also in the inner parts.
Figure 3
SEM images of (a) pure PPy, (b) PPy-HPC, (c) PPy–Ag2O and (d) PPy-HPC/Ag2O nanocomposites.
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SEM images of (a) pure PPy, (b) PPy-HPC, (c) PPy–Ag2O and (d) PPy-HPC/Ag2O nanocomposites.

Figure 3
SEM images of (a) pure PPy, (b) PPy-HPC, (c) PPy–Ag2O and (d) PPy-HPC/Ag2O nanocomposites.
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SEM images of (a) pure PPy, (b) PPy-HPC, (c) PPy–Ag2O and (d) PPy-HPC/Ag2O nanocomposites.

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Effect of pH

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According to the SEM micrographs of the composites, PPy-HPC/Ag2O had the most uniform particular shape and characteristics among the synthesized composites. Thus, the adsorption experiments were carried out using this adsorbent. The pH values of the aqueous solution are an important controlling parameter in the adsorption process, which affects the surface charge of the adsorbent and the ionization of the functional groups onto the adsorbent surfaces (Karthikeyan et al. 2021).

To evaluate the effect of pH on the adsorption, the experiments were carried out at different pH values with an initial metal ion concentration of 50 mg/L. About 0.3 g of adsorbent was added to 30 mL of the metal ion solution and shaken for 45 min at room temperature. Finally, the sorbate was filtered using filter papers and the concentration of heavy metal ions was determined. Figure 4 illustrates the influence of solution pH on the removal of Pb(II) and Co(II) ions using PPy-HPC/Ag2O at different pH conditions ranging from 2 to 7. The removal of Pb(II) and Co(II) ions increased with the increase in pH, and the maximum removal of Pb(II) and Cd(II) ions occurred at pH 6 and 5, respectively. However, experiments were not carried out beyond pH 7 due to the formation of lead hydroxide and cobalt hydroxide, which may result in inconclusive results.
Figure 4
Effect of pH on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II), contact time and the amount of adsorbent were 50 mg/L, 45 min and 0.3 g, respectively).
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Effect of pH on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II), contact time and the amount of adsorbent were 50 mg/L, 45 min and 0.3 g, respectively).

Figure 4
Effect of pH on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II), contact time and the amount of adsorbent were 50 mg/L, 45 min and 0.3 g, respectively).
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Effect of pH on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II), contact time and the amount of adsorbent were 50 mg/L, 45 min and 0.3 g, respectively).

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The increase in Pb(II) and Co(II) removal with the increase in pH can be explained based on H+ ion concentration. At lower pH values, the concentration of H+ ions is higher. The adsorption sites become protonated and competition also exists between the metal ions and H+ ions, which causes the decrease of Pb(II) and removal of Co(II) ions. At higher pH values, the concentration of H+ ions is relatively less. Thus, the affinity for the Pb(II) and Co(II) ions to chelate with the PPy-HPC/Ag2O has been increased without any competition for the adsorption sites, which causes an increase in removal percentage.

Effect of contact time

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The adsorption of Pb(ІІ) and Co(ІІ) ions using the PPy-HPC/Ag2O adsorbent as a function of contact time was investigated at room temperature. Initial metal ion concentration of 50 mg/L and adsorbent dose of 0.3 g in 30 mL of metal ion solution at optimum pH (6 for Pb and 5 for Co) were used. The obtained results are demonstrated in Figure 5. As can be seen at the initial stage of adsorption, metal ion removal has a faster rate due to more active site accessibility and then a gradually slower rate until metal ion uptake becomes almost constant after 30 min. Hence, this time can be chosen as the equilibrium time for further experiments.
Figure 5
Effect of contact time on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II) and the amount of adsorbent were 50 mg/L and 0.3 g, respectively).
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Effect of contact time on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II) and the amount of adsorbent were 50 mg/L and 0.3 g, respectively).

Figure 5
Effect of contact time on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II) and the amount of adsorbent were 50 mg/L and 0.3 g, respectively).
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Effect of contact time on the removal efficiency with PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ions for Pb(II) and Co(II) and the amount of adsorbent were 50 mg/L and 0.3 g, respectively).

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Adsorption isotherms

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The adsorption isotherm for the removal of Pb(ІІ) and Co(ІІ) ions was studied using concentration levels of 25, 50, 75, 100, 150 and 200 mg/L at an adsorbent dosage of 0.3 g/30 mL. It is important to note that the adsorption isotherm is based on the assumption that every adsorption site is equivalent and independent of whether or not adjacent sites are occupied and also the equations show the relationship between metal concentration in solution and the amount of metal sorbed on a specific sorbent at a constant temperature. In order to investigate the adsorption capacities of PPy-HPC/Ag2O for Pb(ІІ) and Co(ІІ), the most commonly used isotherm equations, namely the Langmuir, Freundlich and Temkin isotherms, were taken to analyze the experimental data (Freundlich 1907; Langmuir 1918; Temkin & Pyzhev 1940). The Langmuir (Equation (3)), Freundlich (Equation (4)) and Temkin (Equation (5)) isothermal models can be expressed as follows:
(3)
(4)
(5)
where qe is the amount of metal adsorbed per specific amount of adsorbent (mg/g), KL is the Langmuir constant, Ce is the equilibrium concentration of the solution (mg/L), qm is the maximum amount of metal ions required to form a monolayer (mg/g), KF is the Freundlich adsorption isotherm constant (L/g) and n is adsorption intensity, and bt is the heat of the adsorption (J/mol) and at is the equilibrium binding constant.
The linear plots of adsorption isotherms using the PPy-HPC/Ag2O nanocomposite for Pb(II) and Co(II) are shown in Figures 6 and 7, respectively. The Langmuir and Freundlich adsorption constants evaluated from the isotherms are presented in Table 1. Based on R2 values, it can be seen that the adsorption for Pb(II) and Co(II) was well fitted with that of the Langmuir model, indicating that adsorption was monolayer chemisorption. The maximum theoretical adsorption capacity of PPy-HPC/Ag2O for Pb(II) and Co(II) was 14.947 and 15.504 mg/g, respectively. The values of n between 1 and 10 (1/n less than 1) represent a favorable adsorption. As can be seen in Table 1, the n values for the Freundlich model were found to be greater than 1 for both adsorption experiments, indicating a strong interaction between nanocomposites and Pb(II) and Co(II). In addition, the high R2 value of the Temkin isotherm suggests that the model fits the data well.
Table 1

Langmuir, Freundlich and Temkin isotherm model constants of Pb(II) and Co(II) adsorption onto the PPy-HPC/Ag2O nanocomposites

Adsorption isothermConstantsPb(ІІ)Co(ІІ)
Langmuir qm (mg/g) 14.947 15.504 
KL (L/mg) 0.092 0.053 
R2 0.9719 0.9765 
Freundlich KF (L/g) 2.351 1.557 
n 2.343 1.98 
R2 0.9247 0.9596 
Temkin at 0.87 0 
bt 2.8038 2.7043 
R2 0.9627 0.9529 
Adsorption isothermConstantsPb(ІІ)Co(ІІ)
Langmuir qm (mg/g) 14.947 15.504 
KL (L/mg) 0.092 0.053 
R2 0.9719 0.9765 
Freundlich KF (L/g) 2.351 1.557 
n 2.343 1.98 
R2 0.9247 0.9596 
Temkin at 0.87 0 
bt 2.8038 2.7043 
R2 0.9627 0.9529 
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Figure 6
Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Pb(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Pb(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).
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Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Pb(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Pb(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).

Figure 6
Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Pb(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Pb(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).
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Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Pb(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Pb(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).

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Figure 7
Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Co(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Co(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).
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Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Co(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Co(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).

Figure 7
Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Co(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Co(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).
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Langmuir (a), Freundlich (b) and Temkin (c) adsorption isotherms' linear curve fitting of Co(II) on PPy-HPC/Ag2O (experimental conditions: initial concentrations of heavy metal ion for Co(II) and the amount of adsorbent were 25, 50, 75, 100, 150, 200 mg/L and 0.3 g, respectively).

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Adsorption kinetics

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The adsorption kinetics provide essential information about the solute uptake rate and the reaction pathways. In order to evaluate the kinetic parameters, the pseudo-first-order kinetic model and the pseudo-second-order kinetic model were applied (Mthombeni et al. 2016). The pseudo-first-order kinetic model can be expressed as Equation (6):
(6)
where K1 is the pseudo-first-order rate constant (1/min), t is the contact time (min), and qe and qt (mg/g) are the equilibrium adsorption capacity and adsorption uptake at time t, respectively. The pseudo-second-order kinetic model can be expressed as Equation (7):
(7)
where K2 is the pseudo-second-order rate constant (g/mg·min). The effect of mixing time on the removal of Pb(II) and Co(II) was conducted by adding 0.3 g of adsorbent into a 30 mL solution of 50 mg/L metal ion concentration with mixing times of 3, 5, 8, 10, 15, 20, 30, 40, 60, 80, 100, 120, 150 and 200 min at room temperature. The effect of contact time is shown in Figure 8. It can be seen that the adsorbed amounts increase sharply and reach the equilibrium after 30 min. K1, K2 and qe can be determined from the slope and the intercept of the corresponding linear plots of log (qe − qt) versus t and t/qt versus t and the results are listed in Table 2. It can be seen that all correlation coefficients (R2) of the pseudo-second-order kinetics for Pb(II) and Co(II) on PPy-HPC/Ag2O are higher than those of the pseudo-first-order kinetic model in all the studied experiments. In addition, the values obtained by the pseudo-second-order model are closer to those of the values, indicating that the pseudo-second-order model was better at describing the sorption system.
Table 2

Kinetic parameters of Pb(II) and Co(II) adsorption onto the PPy-HPC/Ag2O nanocomposites

Kinetic modelsParametersPb(ІІ)Co(ІІ)
Pseudo-first-order K1 (1/min) 0.019 0.018 
R2 0.4808 0.4249 
Pseudo-second-order K2 (g/mg·min) 0.026 0.044 
qe (mg/g) 4.926 4.537 
qe, experimental 4.74 4.45 
R2 0.9957 0.9983 
Kinetic modelsParametersPb(ІІ)Co(ІІ)
Pseudo-first-order K1 (1/min) 0.019 0.018 
R2 0.4808 0.4249 
Pseudo-second-order K2 (g/mg·min) 0.026 0.044 
qe (mg/g) 4.926 4.537 
qe, experimental 4.74 4.45 
R2 0.9957 0.9983 
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Figure 8
Effect of adsorption time on the adsorption capacity with PPy-HPC/Ag2O.
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Effect of adsorption time on the adsorption capacity with PPy-HPC/Ag2O.

Figure 8
Effect of adsorption time on the adsorption capacity with PPy-HPC/Ag2O.
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Effect of adsorption time on the adsorption capacity with PPy-HPC/Ag2O.

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Thermodynamic study

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The temperature of 50 mg/L heavy metal ion solution was changed (25, 35 and 50 °C) during adsorption experiments (for the optimum experimental conditions including pH and contact time) to study the thermodynamics of Pb(II) and Co(II) ion adsorption on the PPy-HPC/Ag2O nanocomposite. The thermodynamic parameters, such as Gibbs free energy changes (ΔG), entropy change (ΔS) and enthalpy change (ΔH), were calculated from Equations (8) and (9):
(8)
(9)
where R is the ideal gas constant (8.314 J/mol·K), T (K) is the absolute temperature and K is the thermodynamic equilibrium constant (Javadian 2014). K is equal to the ratio of qe to ce as in Equation (10):
(10)

The obtained values for ΔG, ΔH and ΔS are listed in Table 3. The negative values of ΔG at different temperatures indicate that the removal of Pb(II) and Co(II) ions by PPy-HPC/Ag2O is a spontaneous process and the spontaneity increases with temperature. The positive values of ΔH for all studied experiments confirmed the endothermic nature of the sorption process and pointed out that the adsorption is more active at higher temperatures. In addition, the positive values of ΔS reveal the possibility of increased randomness at the solid and liquid interface during the Pb(II) and Co(II) ion sorption on PPy-HPC/Ag2O. The results acquired from thermodynamics, showing that heat promotes the adsorption of Pb(II) and Co(II) ions on the PPy-HPC/Ag2O nanocomposite, are consistent with those obtained in the adsorption isotherm and adsorption kinetics investigations.

Table 3

Thermodynamic parameters of Pb(II) and Co(II) adsorption onto the PPy-HPC/Ag2O nanocomposites

Thermodynamic parametersPb(ІІ)Co(ІІ)
ΔG (kJ/mol) T = 298 (K) −6.73 −5.13 
T = 308 (K) −7.23 −6.05 
T = 323 (K) −8.26 −6.706 
ΔH (kJ/mol) 11.72 13.08 
ΔS (kJ/mol·K) 0.0618 0.0615 
Thermodynamic parametersPb(ІІ)Co(ІІ)
ΔG (kJ/mol) T = 298 (K) −6.73 −5.13 
T = 308 (K) −7.23 −6.05 
T = 323 (K) −8.26 −6.706 
ΔH (kJ/mol) 11.72 13.08 
ΔS (kJ/mol·K) 0.0618 0.0615 
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Effect of the initial concentration of metal ions on the adsorption

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The concentrations of 30, 60, 100 and 150 (mg/L) of Pb(II) and Co(II) ions from the aqueous solution using composites at the optimum experimental conditions including pH and contact time were investigated. The amount of adsorbent was adjusted to 0.3 g in a 30 mL solution. Tables 4 and 5 show the effect of the initial concentration of Pb(II) and Co(II) on the removal percentage of nanocomposites, respectively.

Table 4

The effect of the initial concentration of Pb(ІІ) ion on the removal efficiency

Type of adsorbent
Initial concentration: 30 (mg/L)
Initial concentration: 60 (mg/L)
Initial concentration: 100 (mg/L)
Initial concentration: 150 (mg/L)
Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)
PPy 6.03 79.9 8.47 85.88 25.09 74.91 48.71 67.52 
PPy-HPC 4.98 83.4 5.73 90.45 26.17 73.83 42.18 71.88 
PPy/Ag2O 3.92 86.93 4.35 92.75 23.19 76.81 44.11 70.6 
PPy-HPC/Ag2O 2.48 91.73 3.4 94.3 21 79 39.5 73.66 
Type of adsorbent
Initial concentration: 30 (mg/L)
Initial concentration: 60 (mg/L)
Initial concentration: 100 (mg/L)
Initial concentration: 150 (mg/L)
Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)
PPy 6.03 79.9 8.47 85.88 25.09 74.91 48.71 67.52 
PPy-HPC 4.98 83.4 5.73 90.45 26.17 73.83 42.18 71.88 
PPy/Ag2O 3.92 86.93 4.35 92.75 23.19 76.81 44.11 70.6 
PPy-HPC/Ag2O 2.48 91.73 3.4 94.3 21 79 39.5 73.66 
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Table 5

The effect of the initial concentration of Co(ІІ) ion on the removal efficiency

Type of adsorbentInitial concentration: 30 (mg/L)
Initial concentration: 60 (mg/L)
Initial concentration: 100 (mg/L)
Initial concentration: 150 (mg/L)
Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)
PPy 8.22 72.6 10.38 82.7 26.39 73.61 52.26 65.16 
PPy-HPC 6.07 79.76 8.45 85.91 29.7 70.3 53.75 64.16 
PPy/Ag2O 4.91 83.63 7.19 88.01 27.14 72.86 49.07 67.28 
PPy-HPC/Ag2O 3.7 87.66 6.2 89.66 23.56 76.44 44.16 70.56 
Type of adsorbentInitial concentration: 30 (mg/L)
Initial concentration: 60 (mg/L)
Initial concentration: 100 (mg/L)
Initial concentration: 150 (mg/L)
Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)Final concentration (mg/L)Removal percentage (%)
PPy 8.22 72.6 10.38 82.7 26.39 73.61 52.26 65.16 
PPy-HPC 6.07 79.76 8.45 85.91 29.7 70.3 53.75 64.16 
PPy/Ag2O 4.91 83.63 7.19 88.01 27.14 72.86 49.07 67.28 
PPy-HPC/Ag2O 3.7 87.66 6.2 89.66 23.56 76.44 44.16 70.56 
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As can be seen, the removal percentage of Pb(II) and Co(II) increases with the initial concentration of metal ions for all of the composites and the maximum value was obtained at about 60 mg/L. The increased number of metal ions in the solution increases the opportunity for the active group to bind with the metal ion, which increases the removal efficiency. Then by increasing the initial concentration of metal ions in the aqueous solution, the removal percentage was reduced. At high initial concentrations, the absorbent surfaces become saturated with the ions and the residual metal ion concentration in the solution is increased. It is noted that the removal efficiency of PPy-HPC for Pb(II) and Co(II) ions is more significant than that of pure PPy, indicating an efficient modification of PPy with HPC. By giving more attention to Tables 4 and 5, it is observed that silver oxide effectively influences the removal percentage. The particle size of the composite was decreased by adding Ag2O. As a result, the total surface of the adsorbent increased and the removal percentage also increased.

To better understand these results, Brunauer–Emmett–Teller (BET) measurements were performed on the surface of the composites. The nanocomposite of PPy-HPC/Ag2O showed a larger surface area of 23.47 m2/g than those of the pure PPy and composite of PPy/Ag2O, which were 6.13 and 13.08 m2/g, respectively. An increase in the surface area agrees with the reduction in the particle size as seen in the SEM image of PPy-HPC/Ag2O. So, it can be confirmed that increasing the PPy-HPC/Ag2O surface leads to an improvement in the percentage removal performance.

Adsorption mechanism

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There are two different mechanisms in the adsorption of heavy metal ions on PPy-HPC/Ag2O: (i) physical adsorption on the surface of PPy-HPC/Ag2O and (ii) chemical adsorption through interactions of PPy molecules with Pb(II) and Co(II) ions. Polypyrrole composites have spherical and porous structures with a high specific surface area. Therefore, metal ions can diffuse through the adsorbent porosity and adsorb on the surface of the composites. Another probable adsorption mechanism occurring on the surface of PPy/Ag2O is the electrostatic interaction of the hydroxyl groups or chelation with N atoms situating in the PPy matrix and metal ions. It can be anticipated that the interactions between ions and the PPy matrix are a result of hybridization between a non-bonding pair of nitrogen atoms of polypyrrole and the empty orbital of metal ions such as Pb(ІІ) (Behera et al. 2022; Shen et al. 2022). Furthermore, the nitrogen species of PPy play crucial major roles in the reduction process of the polymer–metal interface.

Desorption and reusability

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The regeneration of a nanocomposite is an important aspect because it concerns the operating cost of the adsorption process. Desorption experiments were conducted to regenerate the Pb(II)- and Co(II)-loaded PPy-HPC/Ag2O composite for the reuse of the adsorbent. Initially for the adsorption of Pb(II) and Co(II), 0.1 g of PPy-HPC/Ag2O was treated with 50 mL of 100 ppm Pb(II) and Co(II) solutions at pH 6 and 5, respectively. Desorption of Pb(II)- and Co(II)-loaded PPy-HPC/Ag2O adsorbent was done using 50 mL of the 0.5 M NaOH solution. The NaOH solution was chosen since it has the capability of reducing the electrostatic attraction and decreasing the stability of the polypyrrole chain. The regenerated PPy-HPC/Ag2O was examined for five successive adsorption–desorption cycles to verify the reusability of the adsorbent (Figure 9). The percentage of adsorption of the recycled adsorbent was 98.1% and 96.8% in the first cycle for Pb(II) and Co(II), respectively. From the fourth cycle, the removal efficiency starts decreasing. This is due to the imperfect desorption of metal ions. This result suggested that the PPy-HPC/Ag2O composite can be successfully reused for three successive adsorption–desorption cycles with the lowest possible reduction of its removal efficiency. The results confirm that the PPy-HPC/Ag2O nanocomposite can be used as a high-performance and recyclable adsorbent for the removal of heavy metal ions.
Figure 9
Effect of regeneration of adsorbent on removal efficiency.
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Effect of regeneration of adsorbent on removal efficiency.

Figure 9
Effect of regeneration of adsorbent on removal efficiency.
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Effect of regeneration of adsorbent on removal efficiency.

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Comparison with other adsorbents and different methods

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The maximum sorption ability has an important role in the removal of Pb(II) and Co(II). The adsorption capacity of various adsorbents, especially adsorbents containing Ag2O, was compared with previous studies. Data in Table 6 confirm that the conducting polymer PPy was a rich material for the removal of heavy metal ions. Also, Ag2O enables a platform for the incorporation of PPy by which the adsorption capacity of the nanocomposite is increased. The batch adsorption experiment has more remarkable properties than previously reported methods for the removal of Pb(II) and Co(II) ions (Table 7). The advantage of the batch adsorption experiment method is its versatility, which saves time for the adsorption of heavy metal ions with a high conversion percentage and requires less additional and auxiliary equipment.

Table 6

Comparison of adsorption capacities of different adsorbents

AdsorbentMetal ionsAdsorption capacity (mg/g)Reference
TiO2/Ag2O nanoadsorbents Co 0.242 Behera et al. (2022)  
Oil palm bio-waste/MWCNT reinforced polyvinyl alcohol (PVA) hydrogel composites Pb 30.031 Mahmoud et al. (2018)  
Chitosan/PVA beads Pb 9.48 Zulfiqar et al. (2020)  
Torrefied biomass Pb 30.0 Predescu et al. (2019)  
Rose leaf with polypyrrole Pb 1.33 Demey et al. (2019)  
TiO2/poly(acrylamide–styrene sodium sulfonate) Co 100.9 Canoluk & Gursoy (2017)  
Kaolinite modified with tetrabutylammonium bromide Co 9 Borai et al. (2015)  
PPy-HPC/Ag2O nanocomposite Pb 14.947 This work 
AdsorbentMetal ionsAdsorption capacity (mg/g)Reference
TiO2/Ag2O nanoadsorbents Co 0.242 Behera et al. (2022)  
Oil palm bio-waste/MWCNT reinforced polyvinyl alcohol (PVA) hydrogel composites Pb 30.031 Mahmoud et al. (2018)  
Chitosan/PVA beads Pb 9.48 Zulfiqar et al. (2020)  
Torrefied biomass Pb 30.0 Predescu et al. (2019)  
Rose leaf with polypyrrole Pb 1.33 Demey et al. (2019)  
TiO2/poly(acrylamide–styrene sodium sulfonate) Co 100.9 Canoluk & Gursoy (2017)  
Kaolinite modified with tetrabutylammonium bromide Co 9 Borai et al. (2015)  
PPy-HPC/Ag2O nanocomposite Pb 14.947 This work 
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Table 7

Comparison of the proposed method with the previously published methods

AdsorbentMetal ionsAdsorption capacity (mg/g)Reference
Almond green hull Co 45.5 Ahmadpour et al. (2009)  
Montmorillonite modified with tetrabutylammonium bromide Co 22.3 Borai et al. (2015)  
MnO2-coated carbon nanotubes Pb 6.7 Bhattacharyya & Gupta (2009)  
Multiwalled carbon nanotubes Pb 1 Yang et al. (2011)  
Area shell biomass Co 11.53 Dahiya et al. (2008)  
Lemon peel adsorbent Co 22 Bhatnagar et al. (2010)  
Polypyrrole-modified magnetic-reduced graphene oxide composites Pb 93 Liu et al. (2020)  
PPy-HPC/Ag2O nanocomposite Co 15.504 This work 
AdsorbentMetal ionsAdsorption capacity (mg/g)Reference
Almond green hull Co 45.5 Ahmadpour et al. (2009)  
Montmorillonite modified with tetrabutylammonium bromide Co 22.3 Borai et al. (2015)  
MnO2-coated carbon nanotubes Pb 6.7 Bhattacharyya & Gupta (2009)  
Multiwalled carbon nanotubes Pb 1 Yang et al. (2011)  
Area shell biomass Co 11.53 Dahiya et al. (2008)  
Lemon peel adsorbent Co 22 Bhatnagar et al. (2010)  
Polypyrrole-modified magnetic-reduced graphene oxide composites Pb 93 Liu et al. (2020)  
PPy-HPC/Ag2O nanocomposite Co 15.504 This work 
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In this study, polymeric nanocomposites (PPy/Ag2O) were synthesized in the presence of HPC as the surfactant to be used for the adsorption of Pb(II) and Co(II) ions from an aqueous medium. The synthesized composites were analyzed by FTIR spectroscopy, XRD and SEM. The results indicated that the Ag2O and HPC influenced the properties of the synthesized nanocomposites. The extra peaks in the XRD pattern of the nanocomposites indicate the presence of Ag2O nanoparticles in the polypyrrole matrix. Batch adsorption experiments were performed for heavy metal ion removal from the aqueous solution. The adsorption characteristics were investigated at different pH values, contact times and initial metal ion concentrations. The results can be summarized as follows:

  • (1) The pH of the solutions was found to be an important factor in the metal adsorption process. The results show that the removal of Pb(II) and Co(II) ions increased with the increase in pH and the maximum removal of Pb(II) and Cd(II) ions occurred at pH 6 and 5, respectively. This is supported by the fact that the surface of PPy-HPC/Ag2O becomes more negative as pH increases. Negative charges on PPy-HPC/Ag2O surfaces can cause more electrostatic attraction of Pb(II) and Co(II) ions.

  • (2) The optimum equilibrium time was achieved within 30 min of contact time for the adsorption of Pb(II) and Co(II) from the aqueous solution.

  • (3) The adsorption isotherms for Pb(II) and Co(II) on PPy-HPC/Ag2O were well fitted with the Langmuir model.

  • (4) The maximum theoretical adsorption capacity for Pb(II) and Co(II) was 14.947 and 15.504 mg/g, respectively.

  • (5) From the kinetic results, it was concluded that the pseudo-second-order kinetic model was the best at describing the adsorption process.

  • (6) The thermodynamic parameters indicated the endothermic and spontaneous nature of the present adsorption process with increased entropy on PPy-HPC/Ag2O.

Based on these results, the polypyrrole (PPy/Ag2O) in the presence of HPC nanocomposites was found to be appropriate for the removal of Pb(II) and Co(II) from aqueous solutions.

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All relevant data are included in the paper or its Supplementary Information.

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The authors declare there is no conflict.

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© 2025 The Authors
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).