Experimental and DFT studies of the removal of pharmaceutical metronidazole from water using polypyrrole
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The polypyrrole (PPy) was used as an adsorbent material for efficient removal of pharmaceutical metronidazole from aqueous solutions. The physiochemical parameters influencing the adsorption process such as adsorbent dose, temperature, pH, initial concentration and contact time were systematically investigated. The optimum adsorption efficiency is achieved at pH 6.17 after 120 min of contact time. In addition, the Langmuir isotherm and pseudo-second-order models were found to explain the metronidazole adsorption process on the PPy. The thermodynamic parameters indicate that the adsorption of metronidazole on the PPy is a spontaneous and exothermic process. The quantum calculations using density functional theory (DFT) was used to confirm the adsorption mechanism of metronidazole on the PPy. The obtained results of the interaction energy indicate that the adsorption was a physical process. The metronidazole was adsorbed by its oxygen atoms on the amine groups of PPy. Finally, the PPy polymer can be used as an efficient adsorbent for removal of pharmaceutical pollutants from wastewater.
KeywordsAdsorption Density functional theory Pharmaceutical metronidazole Polypyrrole Wastewater
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 .
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 . 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 . Furthermore, the metronidazole can be accumulated in the aquatic environment because of its non-biodegradability and high solubility in water . 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  and flocculation . Among these separation technologies, the adsorption is one of the most promising techniques for removal of the pharmaceuticals from aquatic ecosystems . In this regard, the use of effective adsorbent materials is necessary. Various materials such as clays , agricultural waste  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 .
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
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
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 . 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:
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].
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 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
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.
Kinetic parameters of the adsorption of metronidazole on the PPy
k2 (g/mg min)
kp (mg/g min1/2)
Effect of adsorbent dose
Effect of pH
Thermodynamic parameters of the adsorption of metronidazole on the PPy
ΔS° (J/mol K)
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, Freundlich and Temkin isotherm constants for adsorption of metronidazole on the PPy
DFT study of the adsorption of metronidazole on the PPy
Electronic density distribution
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 . This is in good agreement with the experimental results obtained previously.
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.
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.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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