Introduction

Pharmaceutical compounds are part of the so-called emerging contaminations because of their recent interest in environmental studies. The emerging contaminants are well known as toxic and biorefractory compounds, including pharmaceuticals, pesticides, personal care products, endocrine-disrupting chemicals, and other recalcitrant organic substances [1,2,3]. The pharmaceutical compounds include diverse groups, such as antibiotics, hormones and anticancer agents [4, 5]. The hospitals, households and drug industries are the major sources of pharmaceutical contaminants in aquatic systems [6].

The metronidazole is an antiparasitic agent used for the treatment of Giardia lamblia, Trichomonas vaginalis and Helicobacter pylori infections [7, 8]. In addition, this pharmaceutical byproduct is a carcinogenic and mutagenic compound [9]. The genotoxic activity evaluation indicates that the metronidazole can induce DNA damage in human lymphocytes as well as freshwater and marine organisms [10, 11]. Therefore, its presence in water can cause harmful effects for living organisms and human health [12]. Furthermore, the metronidazole can be accumulated in the aquatic environment because of its non-biodegradability and high solubility in water [13]. For these reasons, the removal of this pollutant from wastewater is a major environmental challenge.

In this context, several decontamination methods have been developed to remove emerging contaminants from water including adsorption [14, 15], heterogeneous catalytic ozonation [16] and flocculation [17]. Among these separation technologies, the adsorption is one of the most promising techniques for removal of the pharmaceuticals from aquatic ecosystems [18]. In this regard, the use of effective adsorbent materials is necessary. Various materials such as clays [19], agricultural waste [20] and organic polymers [21, 22] were used as adsorbents for wastewater treatment.

Recently, the organic polymers (e.g., polypyrrole, polyaniline and polythiophene) were attracted considerable attention in various research fields such as gas sensors, solar cells, corrosion protection, and wastewater decontamination. [23,24,25]. The PPy is one of the most promising organic polymers because of its specific properties like chemical stability, biodegradability, non-toxicity, conductivity and ease of synthesis [26, 27]. The use of PPy as a novel alternative adsorbent material for removal of contaminants from water is mainly related to its large amounts of amino groups and redox reversibility [28].

The present study aims to investigate the adsorption of pharmaceutical metronidazole on the polypyrrole (PPy). The PPy was synthesized via chemical oxidative polymerization of pyrrole monomer in aqueous solution. The textural and structural properties of synthesized PPy were characterized using scanning electron microscopy (SEM) and infrared spectroscopy (IR). To find the optimum adsorption conditions, the effects of physiochemical parameters such as pH, adsorbent dose, contact time, initial concentration of metronidazole and temperature were systematically investigated. The kinetics, isotherms and thermodynamics of the metronidazole adsorption process were also studied. In addition, it was necessary to investigate the adsorption mechanism of the metronidazole on the PPy. The density functional theory (DFT) was recently used to identify the adsorption mechanism of contaminants on the conducting polymers in aqueous and gas phases [25, 29,30,31]. In this way, we examined the geometrical and electronic structures of PPy before and after adsorption of metronidazole using DFT to understand the mechanism involved in the adsorption process.

Materials and methods

Reagents

The pyrrole monomer [98% pyrrole (Aldrich)] is distilled prior to polymerization. The ferric chloride [FeCl ·3 6H2O (Aldrich)] with strength of 0.2 M was used as an oxidant agent. The metronidazole was purchased from Sigma-Aldrich as an analytical grade reagent and used as received without further purification. The stock solution of metronidazole is obtained by dissolving 200 mg of metronidazole in 1 L of distilled water. The solutions used in the experiments were obtained by dilutions to the desired concentrations.

Synthesis of PPy

The oxidative polymerization of pyrrole using ferric chloride was carried out according to the reaction illustrated in Fig. 1. A typical synthesis procedure can be described briefly as follows: 775 µl of pyrrole was mixed with 37.5 ml of distilled water. Then, 12.5 ml of aqueous solution containing 0.2 M of FeCl3, 6H2O was gradually added to start the polymerization reaction [32]. The mixture was stirred for 2 h at room temperature. The resulting PPy was filtered on a filter paper and washed several times with distilled water and ethanol to remove the traces of unreacted products (pyrrole monomer and oxidant). Finally, the PPy was dried in an oven at 65 °C.

Fig. 1
figure 1

Simplified polymerization reaction of pyrrole

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Analysis method

All adsorption experiments were conducted in glass beakers containing 50 mL of metronidazole solution with constant agitation. The pH of the solution was adjusted using the solutions of HCl or NaOH. The solubility of metronidazole was mainly related to the pH of the solution. In the present work, we use a concentration of 10 mg/l (1000 times less than the solubility of metronidazole, 10.61 mg/ml at 25 °C) to investigate the effect of pH on the adsorption process [33]. In addition, the complete solubility of the metronidazole in distilled water (over the pH range of 2–12) was confirmed by UV–visible spectrophotometry. The effects of various physiochemical parameters were systematically investigated using one-variable-at-a-time experimental approach. After each adsorption test, the adsorbent was separated by membrane Millipore filters with 0.45 μm of porosity. Then, the analysis of filtrate samples was done using UV–visible spectrophotometry (Spectrophotometer UV 2300 model) at the maximum absorption wavelength of the metronidazole (320.8 nm). The adsorbed amount and adsorption efficiency are calculated by the following equations:

Adsorbed amount in mg/g:

$$Q_{\text{e}} = \left( {(C_{ 0} - C_{\text{e}} ) \times V} \right)/m$$
(1)

Adsorption efficiency:

$$R(\% ) = \left( {(C_{ 0} - C_{\text{e}} )/C_{ 0} } \right) \times 100$$
(2)

where C0 and Ce are the initial and equilibrium concentrations (mg/l). V and m are the volume of the solution (l) and mass of the PPy (g), respectively.

Calculation methodology

The DFT calculations were performed to examine the adsorption mechanisms of metronidazole on PPy. The optimization of the molecular structures of metronidazole, PPy and their complex was performed. The energetic, structural and electronic properties of the studied molecular system at equilibrium were examined by DFT method. All quantum chemical calculations were performed using the functional hybrid B3LYP with 6–31G (d) base set implemented in Gaussian 09 suite of program [34,35,36,37]. The GaussView 05 software was used as an interface to prepare input files and visualize the calculation results. The optimization of molecular structures is confirmed by the absence of imaginary frequencies [38,39,40,41]. To take into account the effects of the solvent (water), all calculations are carried out using the conductor-like polarizable continuum model (CPCM) as solvation model [42, 43].

The calculation of the adsorbate–adsorbent interaction energy is carried out by the following equation:

$$\Delta E_{\text{int}} = E\;({\text{PPy}}) + E\;({\text{metronidazole}}) - E\;({\text{PPy}}/{\text{metronidazole}})$$
(3)

where E (PPy/metronidazole) is the energy of the complex formed between the PPy and metronidazole. E (PPy) and E (metronidazole) are the energies of the PPy and metronidazole, respectively.

The charge transfer (Δq) between the metronidazole and PPy during the adsorption process in aqueous solution is calculated from the redistribution of the charges on the PPy before and after adsorption. The Δq is expressed by the Eq. (4):

$$\Delta q = q\;{\text{PPy}}\;{\text{before}}\;{\text{adsorption}} - q\;{\text{PPy}}\;{\text{after}}\;{\text{adsorption}}$$
(4)

q PPy before adsorption and q PPy after adsorption are total net charges of the PPy before and after adsorption of metronidazole. The total net charge of the PPy was calculated from the sum of net atomic charges (Mulliken charges) of all atoms constituting the PPy.

Results and discussion

Characterization of the PPy

The morphology of the synthesized PPy was characterized using Field Electron and Ion Quanta 200 scanning electron microscope (SEM) instrument. Figure 2a shows the SEM micrograph of the PPy surface. From this result, it was observed that the PPy possesses a granular heterogeneous morphology. In addition, the surface of PPy exhibits a good porosity. These textural proprieties promote the diffusion of adsorbate molecules into the internal surface of PPy. Therefore, the PPy may be an efficient adsorbent material for the removal of pollutants from aqueous solutions.

Fig. 2
figure 2

a SEM micrograph, b FTIR spectrum of the synthesized PPy

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The PPy was analyzed by Fourier transforms infrared spectroscopy (FTIR Vertex 70 spectrometer) over the wavenumber of 400–4000 cm−1. Figure 2b shows the FTIR spectrum of PPy. The broadband observed at the wavenumber of 3443 cm−1 corresponds to the O–H stretching vibrations of the physisorbed water on the PPy surface. The peak at 1518 cm−1 is due to the asymmetric C=C stretching vibrations of the pyrrole ring. The characteristic peak of the C–C stretching vibrations from the quinonoid structure of the pyrrole is at about 1658 cm−1. The peak at 1291 cm−1 is assigned to C–N stretching vibrations of the PPy. The absorption band at 1293 cm−1 is an indication of the in-plane C–H deformation vibrations. The peak at 824 cm−1 can be assigned to C–H out-of-plane deformation. The C–N stretching vibration peak was observed at 1135 cm−1. The results revealed that the synthesized material corresponds well to the PPy [44, 45].

Adsorption of metronidazole on the PPy

Effect of contact time and adsorption kinetics

The effect of contact time on the removal of metronidazole by PPy was studied in the range of 0–240 min to determine the optimal required time to attain the adsorption equilibrium. Figure 3 illustrates the effect of contact time on removal efficiency of the metronidazole. From these results, it was observed that the adsorption of metronidazole was more rapid in the initial stage of the adsorption. Thereafter, the adsorption rate gradually increases with contact time until the adsorption equilibrium was established. The optimum contact time was found to be 360 min. In the beginning, the rapid metronidazole removal rate may be explained by the availability of a large number of the adsorption sites. However, the number of active sites decreases with contact time, leading to a decrease in the adsorption rate. The spectrum of metronidazole was carried out before and after adsorption (Fig. 3). As a result, we did not observe any spectral change in the positions, disappearance or appearance of new peaks. This indicates that the molecular structure of metronidazole has not changed during the adsorption process.

Fig. 3
figure 3

Effect of time on the adsorption of metronidazole on the PPy (adsorbent dose = 0.5 g/l, C0 = 10 mg/l, pH = 6.17 and T = 25 °C)

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To determine the rate-controlling step for the adsorption of metronidazole on the PPy, the pseudo-first-order, pseudo-second-order and intraparticle diffusion models have been applied to fit the experimental data. The linear expression formulas of these kinetic models can be written in the following forms:

  • Pseudo-first-order kinetic model [46]:


    $$\ln \;(q_{\text{e}} - q_{t} ) = \ln q_{\text{e}} - k_{1} t$$
    (5)

  • Pseudo-second-order kinetic model [47]:


    $$t/q_{t} = 1/k_{2} q_{\text{e}}^{2} + t/q_{\text{e}}$$
    (6)

  • Intraparticle diffusion model [48]:


    $$q_{t} = k_{p} t^{1/2} + C$$
    (7)

where qe (mg/g) and qt (mg/g) are the adsorbed quantities at equilibrium and time t, respectively. k1 (min−1), k2 (g/mg min) and kint (mg/g.min1/2) are the pseudo-first-order, pseudo-second-order and intraparticle diffusion constants, respectively.

The linear fitting of the above-presented kinetic models is shown in Fig. 4. The calculated kinetic parameters and the correlation coefficients are reported in Table 1. As seen, the values of the correlation coefficients (R2) indicate that the pseudo-second-order model (R2 = 0.992) showed a better description of the adsorption of metronidazole on the PPy compared to the pseudo-first-order model (R2 = 0.867). Moreover, the adsorption capacity calculated from pseudo-second-order model (qe.cal = 1.01 mg/g) was much closer to the experimental value (qe.exp = 0.95 mg/g). The diffusion mechanism of the metronidazole into PPy was investigated using the intraparticle diffusion model proposed by Weber and Morris [48]. Figure 4c illustrates the fit of the intraparticle diffusion model to the experimental data. From this figure, it was observed that the intraparticle diffusion model plot is a straight line that did not pass through the origin. This result suggests that the intraparticle diffusion is not the only process determining the adsorption of metronidazole on the PPy. Thus, the mass transfer during the adsorption process can be controlled by two mechanisms: (1) the first one represents external surface adsorption (boundary-layer diffusion); and (2) the second one represents the intraparticle diffusion and adsorption equilibrium stage [49,50,51,52].

Fig. 4
figure 4

a Pseudo-first-order, b pseudo-second-order and c intraparticle diffusion plots for adsorption of metronidazole on the PPy

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Table 1 Kinetic parameters of the adsorption of metronidazole on the PPy
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Effect of adsorbent dose

To examine the effect of the adsorbent dosage on the adsorption performance, the PPy dose was varied over the range of 0.05‒0.70 g/l. The obtained results are illustrated in Fig. 5. As can be seen, the removal efficiency of metronidazole increases with the increase of adsorbent dose. Then, the adsorption was not significantly changed when the PPy dose reaches some value. Increase in the adsorption performance with adsorbent dose may be explained by the large adsorbent surface area and availability of more active sites of the PPy. The maximum adsorption efficiency of metronidazole obtained is 89.55% when the PPy dosage is after 0.5 g/l. In terms of maximum adsorption efficiency and cost-effectiveness, 0.5 g/l was selected as the optimum adsorbent dose for the other adsorption experiments.

Fig. 5
figure 5

Effect of the adsorbent dose on the removal of metronidazole by PPy (t = 360 min, C0 = 1 mg/l, pH = 6.17 and T = 25 °C)

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Effect of pH

The pH influence on the metronidazole adsorption into PPy was studied over the pH range from 2 to 12. The pH of the solution of metronidazole was adjusted by adding NaOH or HCl. The mixtures are stirred for 360 min to ensure that the metronidazole was completely soluble [53]. To investigate the variations in surface charge of the PPy as the pH changes, the point of zero charge (pHZPC) was determined by immersion method as reported by Fiol and Villaescusa [54]. The pHZPC of the PPy is about 6.5. This result indicates that the PPy surface is positively charged when the pH solution is less than 6.5. Meanwhile, the PPy surface becomes negatively charged within the solution pH higher than 6.5. In addition, the metronidazole exists mainly in its deprotonated form when the pH is greater than pKa value (2.55) [55], which comprises almost all the region studied. In contrast, the metronidazole was in its protonated form at pH less than pKa. The effect of initial solution pH on metronidazole adsorption onto PPy is illustrated in Fig. 6. The highest adsorption efficiency of metronidazole was found when the pH is below 6. This can be rationalized by the electrostatic attractions between the positively charged PPy surface and the deprotonated metronidazole (negatively charged). However, the decrease in the adsorption efficiency with an increase in pH from 6 to 12 is probably due to the electrostatic repulsions between negatively charged surface of adsorbent and deprotonated metronidazole.

Fig. 6
figure 6

Effect of pH on the removal of metronidazole by PPy (Adsorbent dose = 0.5 g/l, t = 360 min, C0 = 1 mg/l and T = 25 °C)

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Thermodynamic parameters

The thermodynamic parameters (ΔH°, ΔS° and ΔG°) of the metronidazole adsorption on the PPy are related to the distribution coefficient (Kd) of the solute between the adsorbent surface and the solution by the following equations [56]:

$$\Delta G^\circ = - RT\;\ln K_{\text{d}}$$
(8)
$$\ln K_{\text{d}} = \Delta S^\circ /R - \Delta H^\circ /RT$$
(9)

The coefficient of distribution Kd is defined by the equation [57]:

$$K_{\text{d}} = Q_{\text{e}} /C_{\text{e}}$$
(10)

where R (8.314 J/mol K) is the universal gas constant and T is the absolute solution temperature in Kelvin.

The values of thermodynamic parameters were determined graphically from the plot of ln Kd versus 1/T. The obtained data are shown in Table 2. The negative value of the standard enthalpy ΔH° shows that the adsorption is an exothermic process. The values of the standard free enthalpy are negative (ΔG° < 0), demonstrating that the adsorption is a spontaneous phenomenon. In addition, the low values of ΔG° showed that the adsorption is a physisorption process [58]. The positive value of ΔS° indicates randomness at the solid–liquid interface during the adsorption process.

Table 2 Thermodynamic parameters of the adsorption of metronidazole on the PPy
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Adsorption isotherms

The initial concentration is a key parameter that can significantly affect the adsorption process. The effect of the metronidazole concentration on uptake capacity was investigated in the range of 10‒200 mg/l. As can be seen from Fig. 7, the metronidazole adsorption capacity increased with the initial concentration in the low concentration range. Then, the adsorbed amount grew slowly at high concentration to reach a plateau corresponding to the maximum adsorption capacity. This adsorption behavior may be due to the increase in the mass transfer driving force with increasing initial concentration [59]. Furthermore, the adsorption plateau may be explained by the fact that more active adsorption sites were being covered (saturation of the PPy) at high concentration of the metronidazole.

Fig. 7
figure 7

Effect of the initial concentration on the adsorbed amount of metronidazole by PPy (Adsorbent dose = 0.5 g/l, t = 360 min, T = 25 °C and pH = 6.17)

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To obtain information on the distribution mechanism of the metronidazole on the PPy surface, the experimental equilibrium data were analyzed by Langmuir, Freundlich and Temkin isotherm models. The linear equations of the isotherm models are expressed as follows:

Langmuir model:

$$C_{\text{e}} /Q_{\text{e}} = \, 1/\left( {K_{\text{L}} Q_{\text{m}} } \right) \, + C_{\text{e}} /Q_{\text{m}}$$
(11)

Freundlich model:

$$\ln \;Q_{\text{e}} = \, \ln \;K_{\text{f}} + \, \ln \;C_{\text{e}} /n_{\text{f}}$$
(12)

Temkin model:

$$Q_{\text{e}} = B_{\text{T}} \ln \;K_{\text{T}} + B_{\text{T}} \ln \;C_{\text{e}}$$
(13)

where KL is the Langmuir binding constant, Ce (mg/l) is the equilibrium concentration, Qe (mg/g) is the adsorbed amount at equilibrium, Qm (mg/g) is the maximum adsorption capacity, Kf and n are the Freundlich empirical constants, KT and B are the Temkin Constants.

The separation factor is a dimensionless constant that allows predicting the feasibility of the adsorption process [60]. From Langmuir isotherm, the separation factor (RL) is calculated using the equation:

$$R_{\text{L}} = \frac{1}{{1 + K_{\text{L}} C_{0} }}$$
(14)

where C0 is the initial concentration of metronidazole and KL is the Langmuir binding constant. If RL > 1, this indicates that the adsorption is not favorable. If 0 < RL < 1, this suggests that the adsorption is favorable. If RL = 1, it means linear adsorption.

The graphical presentations of the Langmuir, Freundlich and Temkin adsorption isotherms are represented in Fig. 8. The values of the equilibrium parameters and correlation coefficients (R2) generated by linear regression analysis are given in Table 3. As can be seen from Table 3, the experimental data were best fitted to the Langmuir isotherm model when the correlation coefficient values (R2) are compared. In addition, the maximum adsorption capacity calculated by Langmuir isotherm (5.05 mg/g) is close to that obtained experimentally (5.36 mg/g). Moreover, the value of the separation factor (0 ˂ RL ˂ 1) indicates that the adsorption of metronidazole into PPy is favorable [61]. The value of 1/n (= 0.365) indicates high adsorption affinity between PPy and metronidazole. Thus, the metronidazole adsorption on the PPy is a monolayer process that occurs on the identical active sites.

Fig. 8
figure 8

a Langmuir, b Freundlich and c Temkin isotherm plots for the adsorption of metronidazole on the PPy

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Table 3 Langmuir, Freundlich and Temkin isotherm constants for adsorption of metronidazole on the PPy
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DFT study of the adsorption of metronidazole on the PPy

Electronic density distribution

The distribution of electron density on the optimized geometries of the adsorbate and adsorbent molecules was calculated to identify the nucleophilic and electrophilic attack sites. Figure 9 illustrates the MEP (molecular electrostatic potential) of PPy and the metronidazole. The results obtained show that the oxygenated groups of metronidazole have a high negative electron density and delocalized on these functional groups (red color). This indicates that these groups may be considered as nucleophilic attack sites for the metronidazole molecules. On the other hand, the amine functional groups of tripyrrole lead to a high positive electron density (blue color). This suggests that the amine groups are the electrophilic attack sites for the PPy. According to these results, the adsorption mechanism of metronidazole on the PPy can be carried out through interactions between the nucleophilic (containing oxygen atoms) and electrophilic (–NH–) groups.

Fig. 9
figure 9

Distribution of the electron density on the optimized molecular geometries of the PPy (tripyrrole) and metronidazole

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Charge transfer

The charge transfer phenomenon between the molecules at the solute/adsorbent interface is examined by analysis of the net atomic charges of the PPy before and after complexation/adsorption with metronidazole (Table S1). The results were obtained after optimization of the complex formed between tripyrrole and metronidazole. The redistribution of net charges indicates that the metronidazole loses electrical charges of the order of 0.137e. This loss is transferred to the PPy during the adsorption process. In addition, this low electronic density suggests that the adsorption of metronidazole on the PPy is a physisorption type [62]. This is in good agreement with the experimental results obtained previously.

Intermolecular distances

Figure 10 shows the optimized molecular geometry of the PPy/metronidazole complex. The distances between the two molecules were calculated. The obtained results indicate that there is a short distance (1.795 Å) between the oxygen atom (28O) of metronidazole and the hydrogen (7H) substituted on the nitrogen atom of the PPy. According to International Union of Pure and Applied Chemistry, hydrogen bonds were defined as the electrostatic donor–acceptor interactions between a hydrogen atom (attached to an electronegative atom) and an electronegative atom [63]. The lengths of hydrogen bonds are typically between 1.4 and 3.3 Å. Thus, the intermolecular interactions between the metronidazole and PPy lead to the formation of hydrogen bonds during the adsorption process. This suggests that the adsorption mechanism is essentially a physical process, which is in good adequacy with above-presented experimental results.

Fig. 10
figure 10

Optimized molecular geometry of the complex formed between tripyrrole and metronidazole

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Interaction energy

The calculation of the formation/interaction energies (ΔEint) is very important to study the interactions between the adsorbent material and adsorbate molecule to evaluate the stability of the obtained complex [64, 65]. The value of the interaction energy during the formation of PPy/metronidazole system is equal to − 0.163 u.a. This result confirms that metronidazole molecules present a good affinity on the PPy. In addition, the low value of ΔEint shows that metronidazole adsorbs physically on the PPy surface. This confirms that the adsorption process is favorable.

Conclusion

The PPy was synthesized and used as an adsorbent for removal of metronidazole from aqueous solutions. The effect of contact time on the adsorption of metronidazole on the PPy shows that the equilibrium time does not exceed 360 min. The maximum adsorption efficiency was obtained for a PPy dosage of 0.5 g/l, while the optimum agitation time and pH were 360 min and 6.17, respectively. The adsorption kinetics data were best described by the pseudo-second-order model. The equilibrium adsorption data were found to agree best with the Langmuir model. The thermodynamic parameters suggest that the adsorption is a spontaneous and exothermic process. The quantum chemical calculations using DFT method showed that the adsorption mechanism of metronidazole on the PPy was mainly governed by physical interactions at solid/liquid interface. Overall, the PPy can be used as an eco-friendly and effective adsorbent material for the removal of metronidazole and eventually other pharmaceutical pollutants from the hospital and municipal wastewaters.