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Voltammetric determination of organic nitrogen compounds in environmental samples using carbon paste electrode modified with activated carbon
Microchemical Journal 148 (2019) 66–72
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Voltammetric determination of organic nitrogen compounds in environmental samples using carbon paste electrode modified with activated carbon
T
Meryene de C. Teixeiraa, Fabiana S. Felixa, Sérgio S. Thomasia, Zuy M. Magriotisa, ⁎ Josiane M. da Silvaa, Leonardo L. Okumurab, Adelir A. Saczka, a b
Departamento de Química, Universidade Federal de Lavras, Lavras, MG, Brazil Departamento de Química, Universidade Federal de Viçosa, Viçosa, MG, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Pyridine Quinoline Differential-pulse voltammetry Square-wave voltammetry Textile effluents Fuels
A quick and simple procedure is described to determine pyridine and quinoline using voltammetric methods and carbon paste electrodes modified with activated carbon in the proportion of 50:20:30 w/w graphite:activated carbon:silicon. Studies with cyclic voltammetry indicated that reduction of organic nitrogen molecules is irreversible in ammonium chloride solution acidified to pH 2.5 with Britton-Robinson buffer. Under the best optimum conditions for the differential pulse voltammetry for pyridine and square wave voltammetry for quinoline, the analytical curve was linear from 1.0 × 10−4 to 9.0 × 10−7 mol L−1 for both analytes with limits of detection of approximately 2.0 × 10−7 mol L−1 and limits of quantification of approximately 7.0 × 10−7 mol L−1. The proposed methods were successfully applied to determine pyridine and quinoline in samples of textile effluents and fuel with minimum sample treatment.
1. Introduction Organic nitrogen compounds are considered as environmental pollutants, since the majority of such compounds are carcinogenic and mutagenic for human beings and animals [1]. Among the organic nitrogen compounds, pyridine and quinoline molecules are used for various purposes and are improperly discarded, thereby polluting the environment. Since these molecules are not easily degraded, they are part of a list of 129 substances classified as toxic by the United States Environmental Protection Agency (US EPA) [2]. According to the US EPA, the maximum concentration allowed to discard waste containing pyridine as well as the risk concentration for humans are of 30 μg L−1 and 2130 μg L−1, respectively [2]. Regarding quinoline, high mortality rates in rats were observed for doses between 0 and 800 ppm (0 and 6 mmol L−1) [3]. Despite being toxic, the organic nitrogen molecules are used in the textile industry as additives to set dyes in fibers, as well as part of the raw material to make dyes themselves [2,4]. Products from oil refinery, such as fuels, generate the NOx pollutant, thus representing another source of environmental contamination [5–7]. The US EPA recommends the use of gas chromatography coupled to a mass spectrometer detector (GC–MS) to analyze the organic nitrogen ⁎
compounds [2]. This method was also used in several samples, such as pesticides used in fruits and vegetables [8], and residues present in foods [9,10], besides determination in textile effluents [11] and waste water [12]. Electrochemical techniques have also been applied to remove herbicide in wastewater [13] as well as for the determination of pyridine and quinoline derivates (such as 8-nitroquinoline, azohydroxyquinoline, quinoline yellow, 3,5,6-trichloro-2-pyridyloxyacetic acid) in different samples [14–17]. There are few studies reported relating to the use of voltammetric methods for quantification of pyridine and quinoline [18,19]. Voltammetric techniques are highlighted due to their sensitivity and good limits of detection and quantification, to the possibility of directly determining the concentration of species with minimum sample preparation, and to a smaller quantity of reactants used and waste generated, making analyses fast and simple [20,21]. Voltammetric procedures can represent an interesting alternative to the traditional chromatographic methods for organic nitrogen compound analysis. Sensitivity and/or selectivity of an electrochemical analysis can be improved with the use of modified carbon paste electrodes (MCPEs) [22,23]. Activated carbon (AC) is a modifier that has adsorptive characteristics that contribute to increased sensitivity [24]. Moreover, there
Corresponding author at: Departamento de Química, Universidade Federal de Lavras, Caixa Postal 3037, MG, Brazil. E-mail address: [email protected] (A.A. Saczk).
https://doi.org/10.1016/j.microc.2019.04.038 Received 22 November 2018; Received in revised form 11 April 2019; Accepted 15 April 2019 Available online 19 April 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
Microchemical Journal 148 (2019) 66–72
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are no difficulties in preparing such MCPEs, since the only applied process is homogenization of components. However, procedures to renew the surface of the MCPEs (polish, electrochemical cleaning or paste replacement) must be implemented during analyses. The aim of this study is to develop voltammetric methodologies using carbon paste electrodes modified with activated carbon (MCPE/ AC) to determine pyridine and quinolone in samples from textile effluents and fuels in a fast and precise way, being adequate for routine analyses. Reduction mechanisms of the species of interest are also presented.
2.3. Instruments
2. Materials and methods
3. Results and discussion
2.1. Reactants, solutions and samples
3.1. Studies on cyclic voltammetry
Acetic acid, phosphoric acid and potassium chloride were purchased from Merck. Ammonium chloride, boric acid and sodium dodecyl sulfate were purchased from Synth. Pyridine, quinolone, tetrabutylammonium tetrafluoroborate (TBABF4), cetyltrimethylammonium bromide (CTAB), Triton X-100 (TX-100) and sodium hydroxide were purchased from Sigma-Aldrich. Powder graphite (Synth), liquid silicon (Sigma-Aldrich) and activated carbon (AC – Quimitec) were used to build the MCPEs/AC. Britton-Robinson (BR) 0.02 mol L−1 buffers, at different pH values (2.0–5.5), were prepared by mixing 0.02 mol L−1 phosphoric acid solution to 0.02 mol L−1 acetic acid solution and 0.02 mol L−1 boric acid solution. A 0.10 mol L−1 sodium hydroxide solution was used to obtain the desired pH value. Standard solutions of pyridine and quinoline (1.0 × 10−2 mol L−1) were prepared by dilution of the standard in ultrapure water, and kept in amber flasks under refrigeration. All solutions were prepared in ultrapure water (Milli-Q® Corporation Bedford, EUA resistivity ≥18.2 MΩ cm). The textile effluent sample was provided by the company Papi Têxtil LTDA, located in the city of São Gonçalo do Pará/MG. The gasoline samples were bought at a gas station in the city of Lavras/MG. All samples were collected in amber flasks and kept at 4 °C.
The electrochemical behavior of organic nitrogen molecules was investigated in different KCl, TBABF4 and NH4Cl supporting electrolyte solutions, all at the concentration of 0.5 mol L−1, and in 0.02 mol L−1 BR buffer, at pH ranging from 2.0 to 5.5. The medium must be kept acidic to promote the protonation of quinoline and pyridine, rendering them electroactive [26]. Among the conditions studied, the 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer, at pH 2.5, was chosen to reduce pyridine and quinolone, since it showed the highest intensity values of peak cathodic current [27]. A Potential at Zero Charge (PZC) study, not discussed here, determined that for pH ≤ 3.0 there is a tendency of negative zeta potential values due to the many functional groups, especially the carboxyl ones, which may be deprotonated on the surface of the electrode. This negative surface exerted attraction on the positive charges that compose the structure of the pyridinium and quinolinium ions, contributing to the attraction and subsequent reduction of the analytes. Therefore, PZC results corroborate the pH studies of the BR buffer. Fig. 1 shows the cyclic voltammograms for pyridine (A) and quinoline (B) solutions, both at the concentration 1.0 × 10−2 mol L−1, in 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer. It can be observed in Fig. 1 that the reduction process of the organic nitrogen compounds is irreversible in the potential range used (−1.20 to −1,85 V and −0.5 to −1.40 V vs Ag|AgCl|KCl(sat) for pyridine and quinoline, respectively). The peak potentials for the reduction of pyridine and quinoline are 1.54 V and −1.03 V vs Ag|AgCl|KCl(sat) and are in agreement with data found in the literature [28]. Other cyclic voltammograms (five consecutive scanning cycles) were obtained under the experimental conditions shown in Fig. 1, and a decrease (approximately 80% in the second cycle) in intensity of the cathodic currents for both analytes were observed. Such passivation (inactivation) probably occurred in two ways: the molecule from the reduction product was desorbed but retained between the double electric layer, thereby avoiding the diffusion of other molecules to the surface of the electrode; or the generated product had enough adsorption energy to remain adsorbed in the electrode surface where it underwent reaction [29]. Therefore, after each analysis, it was necessary to clean the electrode. Mass transfer of the pyridine and quinoline
All electrochemical measurements were performed with a Dropsens bipotentiostat/galvanostat μstart 400 Metrohm along with the DropView software (version 2.9). The electrochemical cell (10 mL) consisted of 3 electrodes: MCPE/AC as working electrode, a platinum wire electrode (length 100 mm, Metrohm, Switzerland) as the counter electrode and the reference electrode was Ag|AgCl, 3.0 mol L−1 KCl (Metrohm, Switzerland). A nitrogen flow was applied to the solutions for 10 min to remove electroactive molecular oxygen.
2.2. Preparation of the working electrode In order to make the modified paste, 50% of powder graphite (0–60% m/m), 20% of AC (0–60% m/m) and 30% of silicone (30–40% m/m) were added to a crucible. The silicone was chosen because it provides enhanced consistence to the paste when compared to mineral oil [25]. After being homogenized, the modified carbon paste was introduced into a holder containing a 7.0-cm-long Teflon-coated rod with a small hole of 3.0-mm diameter and internal volume of 14 mm3. The connection was made with a copper wire. SEM images of carbon paste (A) and carbon paste electrodes modified with activated carbon (B) are presented as supplementary material (Fig. S1).
0
(A)
Current ( A)
Current ( A)
0 -300 -600 -900
-1.8
-1.6
-1.4
-1.2
E / V (vs Ag|AgCl|KCl(sat))
Fig. 1. Cyclic voltammograms of the pyridine (A) and quinoline (B) solutions: (a) only 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer at pH 2.5 and (b) solutions of the organic nitrogen compounds at the concentration of 1.0 × 10−2 mol L−1. Working electrode: MCPE/AC. ν = 100 mV s−1. Potential range: from −1.2 to −1.85 V for pyridine and −0.5 to −1.4 V for quinoline.
(B)
-200
-400
-1.4 -1.2 -1.0 -0.8 -0.6
E / V (vs Ag|AgCl|KCl(sat)) 67
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(A)
Current ( A)
Current ( A)
500 0
Current ( A)
2000
-500
0
-2000
-1000
-4000
0
-50
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1.0
2.0
E / V (vs Ag|AgCl|KCl(sat))
-2.0
-1.0
0.0
1.0
2.0
-1.5
E / V (vs Ag|AgCl|KCl(sat))
Current ( A)
Current ( A)
0
-200
-0.5
0.0
0.5
1.0
Fig. 4. Cyclic voltammograms of two consecutive cycles in 0.02 mol L−1 BR buffer supporting electrolyte containing 0.5 mol L−1 NH4Cl, pH 2.5, in the presence of quinoline (1.0 × 10−2 mol L−1). MCPE/AC and ν = 10 mV s−1. P1: quinoline reduction; P2 to P4: possible oxidations and reductions peaks of the product of P1.
(B)
-100
-1.0
E / V (vs Ag|AgCl|KCl(sat))
formation of a radical that might stabilize itself through resonance. Such radical may capture a second electron to form an anion, which is quenched by a proton source, for example, the ammonium ion present in the solution to form the dihydropyridine system. Reduction of dihydropyridine continues until complete formation of piperidine [33]. Piperidine does not undergo oxidation to reestablish aromaticity, which explains the absence of reverse peaks in the voltammogram in Fig. 2A. Four peaks are observed for quinoline in Fig. 2B, two anodic peaks in the potentials 0.0 and 0.75 V vs Ag|AgCl|KCl(sat) and two cathodic peaks in the potentials −0.25 and −1.0 V vs Ag|AgCl|KCl(sat). These peaks are only observed at lower scan rates, showing that more time is necessary for the system to promote reduction and oxidation of the species involved. As seen in Fig. 4, two consecutive cycles were performed to better clarify the redox processes that occur in the quinoline molecule. In cycle 1, an intense peak can be observed at 1.0 V vs Ag|AgCl|KCl(sat) (P1) and another peak with better definition at +0.7 V vs Ag|AgCl|KCl(sat) (P3). In the second cycle, a decrease in the P1 cathodic current and a distortion of P3 are seen, which makes it possible to infer that the oxidation of the species presented in P3 depends directly on the formation of product P1. It can be verified that peaks P2 and P4 are less dependent on the other peaks, since they have the same current intensity in the two cycles. Peak P1 is probably formed by reduction of the quinolinium ion to 1,4-dihydroquinoline. This process may involve the initial formation of a radical followed by the formation of a carbanion that protonates to result in the product. Fig. 5 shows the mechanism proposed to generate P1. Once the 1,4-dihydroquinoline is formed, it may undergo reversible oxidation in the aromatic ring neighboring the ring containing the nitrogen atom. Peak P2 may be formed due to the oxidation that possibly occurs in positions 5 or 7 of the aromatic system. Oxidation must occur in positions 5 or 7 of the aromatic system of 1,4-dihydroquinoline, so that peak P2, whose proposed product was 1,4-dihydroquinoli-7-ol, can be formed. Positions 6 and 8 have higher electron density due to resonance of the only electron pair of the
0
-100
-200 -1.0
0.0
1.0
E / V (vs Ag|AgCl|KCl(sat))
-1.5 -1.0 -0.5 0.0
0.5
1.0
E / V (vs Ag|AgCl|KCl(sat)) Fig. 2. Cyclic voltammograms for (A) pyridine and (B) quinoline: concentration of 1.0 × 10−2 mol L−1 in 0.02 mol L−1 B-R buffer supporting electrolyte containing 0.5 mol L−1 NH4Cl, pH 2.5. In the inset only supporting electrolyte. ν = 10 mV s−1. MCPE/AC. Potential range: from +1.5 to −1.8 V for pyridine and +1.0 to −1.4 V for quinoline.
molecules happened by diffusion, with the contribution of adsorption processes on the surface of the electrode [30]. Other studies on cyclic voltammetry in 0.02 mol L−1 BR/ 0.5 mol L−1 NH4Cl buffer were performed to verify the best electrochemical behavior of the organic nitrogen compounds. For both analytes, the potential range was changed to better observe the reversibility processes with different scan rates (ν) (10, 100 and 200 mV s−1). Fig. 2 shows the voltammograms obtained in such studies for pyridine (A) and quinoline (B) solutions, both at the concentration of 1.0 × 10−2 mol L−1 with ν of 10 mV s−1. Potential range was from +1.5 to −1.8 V vs Ag|AgCl|KCl(sat) for pyridine and from +1,0 to −1,4 V vs Ag|AgCl|KCl(sat) for quinoline. No significant changes were observed for the voltammetric profiles of both analytes at the higher scan rates. For pyridine (Fig. 2A), only one reduction peak at −1.54 V was observed, independently of the scan rates. Therefore, it can be inferred that complete reduction of pyridine occurs, forming piperidine as final product (Fig. 3) [31]. Reduction of pyridine involves the transfer of 2 electrons to form a carbanion, and starts with the formation of a radical through a mechanism similar to the Birch reduction [26]. Addition of the first electron must occur in the position 4 of the aromatic system, once this result in loss of the positive charge of the piridinium ion, leading to the
Fig. 3. Mechanism proposed for electroreduction of pyridine.
68
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H N
H N
H + N
NH 4
Fig. 5. Mechanism proposed for electroreduction of quinoline and formation of P1.
H N
+
- NH 3
1e
1,4 - dihydroquinoline
-
O O
1e
H
H N
-
H N
HO O
HO
O
H N
HO
hydroperoxide
1,4 - dihydroquinoline
nitrogen atom, which probably makes oxidation in such positions difficult. Nevertheless, these oxidations may, in theory, occur in any position. The most probable mechanism of how such oxidation occurs might involve a radical coupling with molecular oxygen yielding a hydroperoxyde. The covalent bonding between the oxygen atoms of the hydroperoxyde may break homoliticaly forming a phenoxyl radical, which stabilizes itself through resonance. The capture of one hydrogen by this radical would generate 1,4-hydroquinolin-7-ol (Fig. 6). Oxidation of 1,4-dihydroquinolin-7-ol results in the P3 peak, yielding 1,4 - dihydroquinolin-4,7-diol. Such reaction is likely to involve a radical coupling with molecular oxygen forming a hydroperoxyde in position 4 of the hydroquinoline ring. This could be due to less steric hindrance and to how easy the intermediate radical is stabilized by resonance. After decomposition of the hydroperoxyde, and later capture of one hydrogen, the neutral compound 1,4-dihydroquinolin-4,7-diol is formed (Fig. 7). The 1,4-dihydroquinolin-4,7-diol (P3) compound underwent reduction, yielding the final product (1,4-dihydroquinolin-4-ol, named as P4, according to the mechanism shown in Fig. 8). Even though the reaction is not thermodynamically favorable, formation of such compound happens due to the donation of electrons by the electrochemical system. At the end of the reduction-oxidation processes described by the aforementioned mechanisms, it was observed that the final product, 1,4–dihydroquinolin-4-ol (peak P4), is different from the initial molecule, quinoline, which confirms an irreversible charge transfer system. The global reaction can be represented by the mechanism shown in Fig. 9. Other studies with cyclic voltammetry and varying ν from 30 to 500 mV s−1 for pyridine and from 10 to 500 mV s−1 for quinoline (keeping the other experimental conditions the same as those of the experiments whose data are shown in Fig. 1) were performed with the MCPE/AC electrode. Peak potential shift was observed for more cathodic regions with increased ν, as expected for irreversible systems. The linear relationship between the cathodic current and ν1/2 complies with the irreversibility criteria [30].
H N HO
1,4 - dihydroquinolin-7-ol
+ OH
OH
1,4 - dihydroquinolin-4-ol
1,4 - dihydroquinolin-4,7-diol
Fig. 8. Mechanism proposed for electroreduction of 1,4-dihydroquinolin-4,7diol and formation of P4.
H N
N
OH quinoline
1,4-dihydroquinolin-4-ol
Fig. 9. Global mechanism of quinoline reduction and formation of 1,4–dihydroquinolin-4-ol.
3.2. Studies on pulse voltammetry 3.2.1. Optimization of parameters With the rise of pulse techniques, it is possible to measure current more favorably as a function of time, for which the capacitive current is already significantly reduced, making it possible to work at low concentrations of analytes in different samples. Therefore, the best conditions to analyze pyridine and quinoline were verified when using a 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer solution, and the differential-pulse voltammetry (DPV) and the square-wave voltammetry (SWV) techniques, respectively. Since it does not present oxidation peaks, pyridine showed the best results with DPV, also because the inverse direction of analyses to quantify the analytes is not considered in such technique [30]. The optimization parameters concerning DPV were studied for a 1.0 × 10−4 mol L−1 pyridine solution in 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer. For such studies, the best conditions were: ν = 50 mV s−1 H N
HO
H N HO
HO HO O
O2
HO
H O O
H N
H N
HO
H N HO
Fig. 6. Mechanism proposed for oxidation of 1,4-dihydroquinoline and P2 formation.
H N
O
hydroperoxide
+ OH
1,4 - dihydroquinolin-4,7-diol
1,4 - dihydroquinolin-7-ol Fig. 7. Mechanism proposed for oxidation of 1,4-dihydroquinolin-7-ol and formation of P3. 69
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(range studied: 10–500 mV s−1), pulse amplitude Epulse = 300 mV (range studied: 10–500 mV), pulse time tp = 1 ms (range studied: 1–20 ms) and scanning step Estep = 5 mV (range studied: 1–50 mV). For the 1.0 × 10−4 mol L−1 quinoline solution, the best conditions for the optimization parameters regarding SWV were: pulse amplitude Eamp = 30 mV (range studied: 1–55 mV), frequency f = 7.0 Hz (range studied: 1–100 Hz) and scanning step Estep = 10 mV (range studied: 1–20 mV).
detection limits (0.25 μg L−1 and 5,05 μg L−1 for pyridine and quinoline, respectively), but under severe reduction conditions in DMF/ TBAF4 electrolyte (−2.52 V for pyridine and −1.95 V for quinoline). 3.2.4. Determining pyridine and quinoline in fuel and textile samples Performance of the MCPE/AC electrode using DPV and SWV to determine pyridine and quinoline, respectively, was checked through analyses of samples of gasoline and textile effluents. It was verified that such samples did not present detectable quantities of analytes and, thus, curves were built with the method of standard addition. In addition, recovering trials were performed with stock solutions of pyridine and quinoline at the concentration of 1.0 × 10−2 mol L−1. When it comes to the gasoline samples, it was necessary to add surfactant to the support electrolyte to enhance sample solubilization. The surfactants were sodium dodecyl sulfate (SDS, anionic), Triton X100 (TX-100, nonionic) and cetyltrimethylammonium bromide (CTAB, cationic). Among the studied surfactants, CTAB did not interfere in the analyses of pyridine and quinoline and favored reduction, since it has NH4+ in its composition. Through voltammetry studies, it was verified that the best CTAB concentration was of 1.0 × 10−5 mol L−1. The standard addition curves and recovered studies were performed under the same experimental conditions as those of the analytical curves (Fig. 12). Fig. 12 shows the DPV voltammograms for the increasing additions of standard solutions of pyridine to textile effluent samples (A) and gasoline (B), for a concentration range of 9.0 × 10−7 to 1.0 × 10−4 mol L−1. The regression equations corresponding to the linear range of concentration for pyridine in textile effluent and gasoline samples were i = 59.40 + 1.22 × 107 c, r = 0.9904, and i = 61.87 + 7.82 × 106 c, r = 0.9901 (Fig. 12), where i is the current (in μA) and c is the pyridine concentration (in μmol L−1). By using the t-student test (confidence interval of 95%) [33] for the values of angular coefficient of the pyridine analytical curves with external standardization (8.88 × 106 μA mol−1 L) and with standard addition (1.22 × 107 μA mol−1 L and 7.82 × 106 μA mol−1 L), no significant differences were observed regarding the analyses of pyridine in textile effluent and gasoline samples. These satisfactory results were also obtained for quinoline in the same samples. Concentrations of 1.0, 10 and 100 mol L−1 were used for the recovered studies for both analytes. The environmental samples presented recovering intervals of 95 to 99% for pyridine and of 96 to 98% for quinoline, and intervals of 92 to 98% were observed for pyridine and quinoline in gasoline samples. These values (satisfactory, considering the low concentration and presence of relatively complex samples) show that the analytes did not undergo major interferences from other species present in the samples used in the experiments herein described.
3.2.2. Studies on the electochemical behavior of organic nitrogen compounds in the presence of NH4Cl Voltammetry studies were performed to verify the importance of the ammonium salt to the composition of the electrolyte solution during the electrochemical reduction of analytes. In order to do so, solutions of KCl, NH4Cl and (NH4)2SO4 were used at the concentration of 0.5 mol L−1, keeping the pH 2.5 0.02 mol L−1 BR buffer solution to ensure an acidic medium. A study with KCl and NH4Cl was performed and great differences regarding the values of peak cathodic currents were observed. To confirm the influence of the cation (NH4+) on the reduction process, the solution of NH4Cl was replaced by a (NH4)2SO4 solution, and no significant differences concerning the reduction peak currents were observed. Such observations can be seen in Fig. 10. The results obtained for pyridine (Fig. 10) were similar to those of quinoline and corroborate the cyclic voltammetry studies (Figs. 2 and 3), in which it was observed that the NH4+ ions are important for the reduction of organic nitrogen compounds. Moreover, when comparing the cathodic currents for pyridine reduction in solutions with NH4Cl and (NH4)2SO4, great variations in this parameter are not observed. Therefore, the replacement of Cl− ions with SO42− did not interfere in the electrochemical processes of the studied molecules. 3.2.3. Analytical curves for pyridine and quinoline After the optimization steps were carried out in 0.02 mol L−1 BR/ 0.5 mol L−1 NH4Cl buffer solution, using DPV for pyridine and SWV for quinoline, different aliquots from stock solutions of 1.0 × 10−2 mol L−1 organic nitrogen compounds were added to the electrolyte solution to determine the linear relationship between the cathodic current signals and the concentrations of analytes. With the results, it was possible to obtain analytical curves at the concentration range between 9.0 × 10−7 and 1.0 × 10−4 mol L−1 for pyridine (Fig. 11A) and quinoline (Fig. 11B). From Fig. 11, the regression equations corresponding to the linear concentration range of pyridine (i = 63.40 + 8.88 × 106 c, with r = 0.9903) and quinoline (i = 4.15 + 3.56 × 106 c, with r = 0.9943) were obtained, where i is the current (in μA), and c is the concentration of analytes in mol L−1. It was possible to perform 74 determinations of pyridine or quinoline by voltammetry for each MCPE/AC assembly. The detection limits estimated [32] were 2.01 × 10−7 mol L−1 (15.87 μg L−1) and 1.98 × 10−7 mol L−1 (25.54 μg L−1), and the quantification ones were 6.69 × 10−7 mol L−1 (52.85 μg L−1) and 6.53 × 10−7 mol L−1 (84.24 μg L−1) for pyridine and quinoline, respectively. Okumura and Stradiotto [21] have also achieved good 0
In this study, carbon paste electrodes modified with activated carbon were prepared and applied to analyze organic nitrogen Fig. 10. (A) Voltammograms with 0.02 mol L−1 BR buffer in the presence of different cations: (a) addition of NH4Cl, (b) addition of pyridine, (c) addition of KCl and pyridine, (d) addition of NH4Cl and pyridine; (B) 0.02 mol L−1 BR buffer containing different anions: (a) addition of NH4Cl, (b) addition of (NH4)2SO4 and pyridine, (c) addition of NH4Cl and pyridine. [Salts]: 0.5 mol L−1, [pyridine] = 1.0 −4 −1 × 10 mol L . MCPE/AC.
0
(A) Current ( A)
Current ( A)
(B)
-200
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-1.8
4. Conclusion
-1.6
-1.4
-1.2
-1.0
E / V (vs Ag|AgCl|KCl(sat))
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-400
-1.8
-1.6
-1.4
-1.2
-1.0
E / V (vs Ag|AgCl|KCl(sat)) 70
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Current ( A)
a -400
j
-600
-1.8 -1.6 -1.4 -1.2 -1.0
E / V (vs Ag|AgCl|KCl(sat))
200
80
8
-80
j
-1.5 -1.0 -0.5 E / V (vs Ag|AgCl|KCl(sat))
40
(B)
0
100 4
Fig. 11. Analytical curves obtained after successive additions of pyridine (A) and quinoline (B) at the concentrations of 9.0 × 10−7, 1.0 × 10−6, 3.0 × 10−6, 6.0 × 10−6, 9.0 × 10−6, 1.0 × 10−5, 3.0 × 10−5, 6.0 × 10−5, −5 −4 −1 9.0 × 10 , 1.0 × 10 mol L . For pyridine: (DPV) ν = 50 mV s−1, Epulse = 300 mV, tp = 1 ms, Estep = 5 mV, and for quinoline: (SWV) Eamp = 30 mV, Estep = 10 mV, f = 7 Hz. MCPE/AC.
a
-40
-120
(A) 0
i ( A)
-200
i ( A)
Current ( A)
300
0
40 60 80 100
5
-1
10 40
60
-1
80 100
CQuinoline ( mol L )
CPyridine ( mol L )
400
a j
-900
200
100
(A) 4
a
-600
-1.6 -1.4 -1.2 -1.0 E / V (vs Ag|AgCl|KCl(sat))
0
-300
8
40
60 -1 80 100
CPyridine ( mol L )
300
i ( A)
-600
Current ( A)
300
i ( A)
Current ( A)
-300
j
-900 -1.6
-1.4
-1.2
-1.0
E / V (vs Ag|AgCl|KCl(sat))
200
100
(B) 0
4
8
40
60
-1
80 100
CPyridine ( mol L )
Fig. 12. Analytical curves obtained after successive additions of pyridine at the concentrations of 9.0 × 10−7 to 1.0 × 10−4 mol L−1 to the samples: (A) textile effluents and (B) gasoline. ν = 50 mV s−1, Epulse = 30 mV, tp = 1 ms, Estep = 5 mV. MCPE/AC.
compounds in environmental samples. The modified electrode favored electrochemical reduction of analytes in an irreversible process with mass transfer through diffusion and contribution of adsorption. The voltammetry methods proposed were satisfactory when compared to the maximum concentration limits of such molecules required by the current legislation.
[9]
[10]
Acknowledgements The authors gratefully acknowledge financial support from the Brazilian agencies FAPEMIG, CNPq and CAPES (Finance Code 001).
[11] [12]
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.04.038.
[13]
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Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Voltammetric determination of organic nitrogen compounds in environmental samples using carbon paste electrode modified with activated carbon
T
Meryene de C. Teixeiraa, Fabiana S. Felixa, Sérgio S. Thomasia, Zuy M. Magriotisa, ⁎ Josiane M. da Silvaa, Leonardo L. Okumurab, Adelir A. Saczka, a b
Departamento de Química, Universidade Federal de Lavras, Lavras, MG, Brazil Departamento de Química, Universidade Federal de Viçosa, Viçosa, MG, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Pyridine Quinoline Differential-pulse voltammetry Square-wave voltammetry Textile effluents Fuels
A quick and simple procedure is described to determine pyridine and quinoline using voltammetric methods and carbon paste electrodes modified with activated carbon in the proportion of 50:20:30 w/w graphite:activated carbon:silicon. Studies with cyclic voltammetry indicated that reduction of organic nitrogen molecules is irreversible in ammonium chloride solution acidified to pH 2.5 with Britton-Robinson buffer. Under the best optimum conditions for the differential pulse voltammetry for pyridine and square wave voltammetry for quinoline, the analytical curve was linear from 1.0 × 10−4 to 9.0 × 10−7 mol L−1 for both analytes with limits of detection of approximately 2.0 × 10−7 mol L−1 and limits of quantification of approximately 7.0 × 10−7 mol L−1. The proposed methods were successfully applied to determine pyridine and quinoline in samples of textile effluents and fuel with minimum sample treatment.
1. Introduction Organic nitrogen compounds are considered as environmental pollutants, since the majority of such compounds are carcinogenic and mutagenic for human beings and animals [1]. Among the organic nitrogen compounds, pyridine and quinoline molecules are used for various purposes and are improperly discarded, thereby polluting the environment. Since these molecules are not easily degraded, they are part of a list of 129 substances classified as toxic by the United States Environmental Protection Agency (US EPA) [2]. According to the US EPA, the maximum concentration allowed to discard waste containing pyridine as well as the risk concentration for humans are of 30 μg L−1 and 2130 μg L−1, respectively [2]. Regarding quinoline, high mortality rates in rats were observed for doses between 0 and 800 ppm (0 and 6 mmol L−1) [3]. Despite being toxic, the organic nitrogen molecules are used in the textile industry as additives to set dyes in fibers, as well as part of the raw material to make dyes themselves [2,4]. Products from oil refinery, such as fuels, generate the NOx pollutant, thus representing another source of environmental contamination [5–7]. The US EPA recommends the use of gas chromatography coupled to a mass spectrometer detector (GC–MS) to analyze the organic nitrogen ⁎
compounds [2]. This method was also used in several samples, such as pesticides used in fruits and vegetables [8], and residues present in foods [9,10], besides determination in textile effluents [11] and waste water [12]. Electrochemical techniques have also been applied to remove herbicide in wastewater [13] as well as for the determination of pyridine and quinoline derivates (such as 8-nitroquinoline, azohydroxyquinoline, quinoline yellow, 3,5,6-trichloro-2-pyridyloxyacetic acid) in different samples [14–17]. There are few studies reported relating to the use of voltammetric methods for quantification of pyridine and quinoline [18,19]. Voltammetric techniques are highlighted due to their sensitivity and good limits of detection and quantification, to the possibility of directly determining the concentration of species with minimum sample preparation, and to a smaller quantity of reactants used and waste generated, making analyses fast and simple [20,21]. Voltammetric procedures can represent an interesting alternative to the traditional chromatographic methods for organic nitrogen compound analysis. Sensitivity and/or selectivity of an electrochemical analysis can be improved with the use of modified carbon paste electrodes (MCPEs) [22,23]. Activated carbon (AC) is a modifier that has adsorptive characteristics that contribute to increased sensitivity [24]. Moreover, there
Corresponding author at: Departamento de Química, Universidade Federal de Lavras, Caixa Postal 3037, MG, Brazil. E-mail address: [email protected] (A.A. Saczk).
https://doi.org/10.1016/j.microc.2019.04.038 Received 22 November 2018; Received in revised form 11 April 2019; Accepted 15 April 2019 Available online 19 April 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.
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are no difficulties in preparing such MCPEs, since the only applied process is homogenization of components. However, procedures to renew the surface of the MCPEs (polish, electrochemical cleaning or paste replacement) must be implemented during analyses. The aim of this study is to develop voltammetric methodologies using carbon paste electrodes modified with activated carbon (MCPE/ AC) to determine pyridine and quinolone in samples from textile effluents and fuels in a fast and precise way, being adequate for routine analyses. Reduction mechanisms of the species of interest are also presented.
2.3. Instruments
2. Materials and methods
3. Results and discussion
2.1. Reactants, solutions and samples
3.1. Studies on cyclic voltammetry
Acetic acid, phosphoric acid and potassium chloride were purchased from Merck. Ammonium chloride, boric acid and sodium dodecyl sulfate were purchased from Synth. Pyridine, quinolone, tetrabutylammonium tetrafluoroborate (TBABF4), cetyltrimethylammonium bromide (CTAB), Triton X-100 (TX-100) and sodium hydroxide were purchased from Sigma-Aldrich. Powder graphite (Synth), liquid silicon (Sigma-Aldrich) and activated carbon (AC – Quimitec) were used to build the MCPEs/AC. Britton-Robinson (BR) 0.02 mol L−1 buffers, at different pH values (2.0–5.5), were prepared by mixing 0.02 mol L−1 phosphoric acid solution to 0.02 mol L−1 acetic acid solution and 0.02 mol L−1 boric acid solution. A 0.10 mol L−1 sodium hydroxide solution was used to obtain the desired pH value. Standard solutions of pyridine and quinoline (1.0 × 10−2 mol L−1) were prepared by dilution of the standard in ultrapure water, and kept in amber flasks under refrigeration. All solutions were prepared in ultrapure water (Milli-Q® Corporation Bedford, EUA resistivity ≥18.2 MΩ cm). The textile effluent sample was provided by the company Papi Têxtil LTDA, located in the city of São Gonçalo do Pará/MG. The gasoline samples were bought at a gas station in the city of Lavras/MG. All samples were collected in amber flasks and kept at 4 °C.
The electrochemical behavior of organic nitrogen molecules was investigated in different KCl, TBABF4 and NH4Cl supporting electrolyte solutions, all at the concentration of 0.5 mol L−1, and in 0.02 mol L−1 BR buffer, at pH ranging from 2.0 to 5.5. The medium must be kept acidic to promote the protonation of quinoline and pyridine, rendering them electroactive [26]. Among the conditions studied, the 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer, at pH 2.5, was chosen to reduce pyridine and quinolone, since it showed the highest intensity values of peak cathodic current [27]. A Potential at Zero Charge (PZC) study, not discussed here, determined that for pH ≤ 3.0 there is a tendency of negative zeta potential values due to the many functional groups, especially the carboxyl ones, which may be deprotonated on the surface of the electrode. This negative surface exerted attraction on the positive charges that compose the structure of the pyridinium and quinolinium ions, contributing to the attraction and subsequent reduction of the analytes. Therefore, PZC results corroborate the pH studies of the BR buffer. Fig. 1 shows the cyclic voltammograms for pyridine (A) and quinoline (B) solutions, both at the concentration 1.0 × 10−2 mol L−1, in 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer. It can be observed in Fig. 1 that the reduction process of the organic nitrogen compounds is irreversible in the potential range used (−1.20 to −1,85 V and −0.5 to −1.40 V vs Ag|AgCl|KCl(sat) for pyridine and quinoline, respectively). The peak potentials for the reduction of pyridine and quinoline are 1.54 V and −1.03 V vs Ag|AgCl|KCl(sat) and are in agreement with data found in the literature [28]. Other cyclic voltammograms (five consecutive scanning cycles) were obtained under the experimental conditions shown in Fig. 1, and a decrease (approximately 80% in the second cycle) in intensity of the cathodic currents for both analytes were observed. Such passivation (inactivation) probably occurred in two ways: the molecule from the reduction product was desorbed but retained between the double electric layer, thereby avoiding the diffusion of other molecules to the surface of the electrode; or the generated product had enough adsorption energy to remain adsorbed in the electrode surface where it underwent reaction [29]. Therefore, after each analysis, it was necessary to clean the electrode. Mass transfer of the pyridine and quinoline
All electrochemical measurements were performed with a Dropsens bipotentiostat/galvanostat μstart 400 Metrohm along with the DropView software (version 2.9). The electrochemical cell (10 mL) consisted of 3 electrodes: MCPE/AC as working electrode, a platinum wire electrode (length 100 mm, Metrohm, Switzerland) as the counter electrode and the reference electrode was Ag|AgCl, 3.0 mol L−1 KCl (Metrohm, Switzerland). A nitrogen flow was applied to the solutions for 10 min to remove electroactive molecular oxygen.
2.2. Preparation of the working electrode In order to make the modified paste, 50% of powder graphite (0–60% m/m), 20% of AC (0–60% m/m) and 30% of silicone (30–40% m/m) were added to a crucible. The silicone was chosen because it provides enhanced consistence to the paste when compared to mineral oil [25]. After being homogenized, the modified carbon paste was introduced into a holder containing a 7.0-cm-long Teflon-coated rod with a small hole of 3.0-mm diameter and internal volume of 14 mm3. The connection was made with a copper wire. SEM images of carbon paste (A) and carbon paste electrodes modified with activated carbon (B) are presented as supplementary material (Fig. S1).
0
(A)
Current ( A)
Current ( A)
0 -300 -600 -900
-1.8
-1.6
-1.4
-1.2
E / V (vs Ag|AgCl|KCl(sat))
Fig. 1. Cyclic voltammograms of the pyridine (A) and quinoline (B) solutions: (a) only 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer at pH 2.5 and (b) solutions of the organic nitrogen compounds at the concentration of 1.0 × 10−2 mol L−1. Working electrode: MCPE/AC. ν = 100 mV s−1. Potential range: from −1.2 to −1.85 V for pyridine and −0.5 to −1.4 V for quinoline.
(B)
-200
-400
-1.4 -1.2 -1.0 -0.8 -0.6
E / V (vs Ag|AgCl|KCl(sat)) 67
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(A)
Current ( A)
Current ( A)
500 0
Current ( A)
2000
-500
0
-2000
-1000
-4000
0
-50
-100
-2.0 -1.0 0.0
1.0
2.0
E / V (vs Ag|AgCl|KCl(sat))
-2.0
-1.0
0.0
1.0
2.0
-1.5
E / V (vs Ag|AgCl|KCl(sat))
Current ( A)
Current ( A)
0
-200
-0.5
0.0
0.5
1.0
Fig. 4. Cyclic voltammograms of two consecutive cycles in 0.02 mol L−1 BR buffer supporting electrolyte containing 0.5 mol L−1 NH4Cl, pH 2.5, in the presence of quinoline (1.0 × 10−2 mol L−1). MCPE/AC and ν = 10 mV s−1. P1: quinoline reduction; P2 to P4: possible oxidations and reductions peaks of the product of P1.
(B)
-100
-1.0
E / V (vs Ag|AgCl|KCl(sat))
formation of a radical that might stabilize itself through resonance. Such radical may capture a second electron to form an anion, which is quenched by a proton source, for example, the ammonium ion present in the solution to form the dihydropyridine system. Reduction of dihydropyridine continues until complete formation of piperidine [33]. Piperidine does not undergo oxidation to reestablish aromaticity, which explains the absence of reverse peaks in the voltammogram in Fig. 2A. Four peaks are observed for quinoline in Fig. 2B, two anodic peaks in the potentials 0.0 and 0.75 V vs Ag|AgCl|KCl(sat) and two cathodic peaks in the potentials −0.25 and −1.0 V vs Ag|AgCl|KCl(sat). These peaks are only observed at lower scan rates, showing that more time is necessary for the system to promote reduction and oxidation of the species involved. As seen in Fig. 4, two consecutive cycles were performed to better clarify the redox processes that occur in the quinoline molecule. In cycle 1, an intense peak can be observed at 1.0 V vs Ag|AgCl|KCl(sat) (P1) and another peak with better definition at +0.7 V vs Ag|AgCl|KCl(sat) (P3). In the second cycle, a decrease in the P1 cathodic current and a distortion of P3 are seen, which makes it possible to infer that the oxidation of the species presented in P3 depends directly on the formation of product P1. It can be verified that peaks P2 and P4 are less dependent on the other peaks, since they have the same current intensity in the two cycles. Peak P1 is probably formed by reduction of the quinolinium ion to 1,4-dihydroquinoline. This process may involve the initial formation of a radical followed by the formation of a carbanion that protonates to result in the product. Fig. 5 shows the mechanism proposed to generate P1. Once the 1,4-dihydroquinoline is formed, it may undergo reversible oxidation in the aromatic ring neighboring the ring containing the nitrogen atom. Peak P2 may be formed due to the oxidation that possibly occurs in positions 5 or 7 of the aromatic system. Oxidation must occur in positions 5 or 7 of the aromatic system of 1,4-dihydroquinoline, so that peak P2, whose proposed product was 1,4-dihydroquinoli-7-ol, can be formed. Positions 6 and 8 have higher electron density due to resonance of the only electron pair of the
0
-100
-200 -1.0
0.0
1.0
E / V (vs Ag|AgCl|KCl(sat))
-1.5 -1.0 -0.5 0.0
0.5
1.0
E / V (vs Ag|AgCl|KCl(sat)) Fig. 2. Cyclic voltammograms for (A) pyridine and (B) quinoline: concentration of 1.0 × 10−2 mol L−1 in 0.02 mol L−1 B-R buffer supporting electrolyte containing 0.5 mol L−1 NH4Cl, pH 2.5. In the inset only supporting electrolyte. ν = 10 mV s−1. MCPE/AC. Potential range: from +1.5 to −1.8 V for pyridine and +1.0 to −1.4 V for quinoline.
molecules happened by diffusion, with the contribution of adsorption processes on the surface of the electrode [30]. Other studies on cyclic voltammetry in 0.02 mol L−1 BR/ 0.5 mol L−1 NH4Cl buffer were performed to verify the best electrochemical behavior of the organic nitrogen compounds. For both analytes, the potential range was changed to better observe the reversibility processes with different scan rates (ν) (10, 100 and 200 mV s−1). Fig. 2 shows the voltammograms obtained in such studies for pyridine (A) and quinoline (B) solutions, both at the concentration of 1.0 × 10−2 mol L−1 with ν of 10 mV s−1. Potential range was from +1.5 to −1.8 V vs Ag|AgCl|KCl(sat) for pyridine and from +1,0 to −1,4 V vs Ag|AgCl|KCl(sat) for quinoline. No significant changes were observed for the voltammetric profiles of both analytes at the higher scan rates. For pyridine (Fig. 2A), only one reduction peak at −1.54 V was observed, independently of the scan rates. Therefore, it can be inferred that complete reduction of pyridine occurs, forming piperidine as final product (Fig. 3) [31]. Reduction of pyridine involves the transfer of 2 electrons to form a carbanion, and starts with the formation of a radical through a mechanism similar to the Birch reduction [26]. Addition of the first electron must occur in the position 4 of the aromatic system, once this result in loss of the positive charge of the piridinium ion, leading to the
Fig. 3. Mechanism proposed for electroreduction of pyridine.
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H N
H N
H + N
NH 4
Fig. 5. Mechanism proposed for electroreduction of quinoline and formation of P1.
H N
+
- NH 3
1e
1,4 - dihydroquinoline
-
O O
1e
H
H N
-
H N
HO O
HO
O
H N
HO
hydroperoxide
1,4 - dihydroquinoline
nitrogen atom, which probably makes oxidation in such positions difficult. Nevertheless, these oxidations may, in theory, occur in any position. The most probable mechanism of how such oxidation occurs might involve a radical coupling with molecular oxygen yielding a hydroperoxyde. The covalent bonding between the oxygen atoms of the hydroperoxyde may break homoliticaly forming a phenoxyl radical, which stabilizes itself through resonance. The capture of one hydrogen by this radical would generate 1,4-hydroquinolin-7-ol (Fig. 6). Oxidation of 1,4-dihydroquinolin-7-ol results in the P3 peak, yielding 1,4 - dihydroquinolin-4,7-diol. Such reaction is likely to involve a radical coupling with molecular oxygen forming a hydroperoxyde in position 4 of the hydroquinoline ring. This could be due to less steric hindrance and to how easy the intermediate radical is stabilized by resonance. After decomposition of the hydroperoxyde, and later capture of one hydrogen, the neutral compound 1,4-dihydroquinolin-4,7-diol is formed (Fig. 7). The 1,4-dihydroquinolin-4,7-diol (P3) compound underwent reduction, yielding the final product (1,4-dihydroquinolin-4-ol, named as P4, according to the mechanism shown in Fig. 8). Even though the reaction is not thermodynamically favorable, formation of such compound happens due to the donation of electrons by the electrochemical system. At the end of the reduction-oxidation processes described by the aforementioned mechanisms, it was observed that the final product, 1,4–dihydroquinolin-4-ol (peak P4), is different from the initial molecule, quinoline, which confirms an irreversible charge transfer system. The global reaction can be represented by the mechanism shown in Fig. 9. Other studies with cyclic voltammetry and varying ν from 30 to 500 mV s−1 for pyridine and from 10 to 500 mV s−1 for quinoline (keeping the other experimental conditions the same as those of the experiments whose data are shown in Fig. 1) were performed with the MCPE/AC electrode. Peak potential shift was observed for more cathodic regions with increased ν, as expected for irreversible systems. The linear relationship between the cathodic current and ν1/2 complies with the irreversibility criteria [30].
H N HO
1,4 - dihydroquinolin-7-ol
+ OH
OH
1,4 - dihydroquinolin-4-ol
1,4 - dihydroquinolin-4,7-diol
Fig. 8. Mechanism proposed for electroreduction of 1,4-dihydroquinolin-4,7diol and formation of P4.
H N
N
OH quinoline
1,4-dihydroquinolin-4-ol
Fig. 9. Global mechanism of quinoline reduction and formation of 1,4–dihydroquinolin-4-ol.
3.2. Studies on pulse voltammetry 3.2.1. Optimization of parameters With the rise of pulse techniques, it is possible to measure current more favorably as a function of time, for which the capacitive current is already significantly reduced, making it possible to work at low concentrations of analytes in different samples. Therefore, the best conditions to analyze pyridine and quinoline were verified when using a 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer solution, and the differential-pulse voltammetry (DPV) and the square-wave voltammetry (SWV) techniques, respectively. Since it does not present oxidation peaks, pyridine showed the best results with DPV, also because the inverse direction of analyses to quantify the analytes is not considered in such technique [30]. The optimization parameters concerning DPV were studied for a 1.0 × 10−4 mol L−1 pyridine solution in 0.02 mol L−1 BR/0.5 mol L−1 NH4Cl buffer. For such studies, the best conditions were: ν = 50 mV s−1 H N
HO
H N HO
HO HO O
O2
HO
H O O
H N
H N
HO
H N HO
Fig. 6. Mechanism proposed for oxidation of 1,4-dihydroquinoline and P2 formation.
H N
O
hydroperoxide
+ OH
1,4 - dihydroquinolin-4,7-diol
1,4 - dihydroquinolin-7-ol Fig. 7. Mechanism proposed for oxidation of 1,4-dihydroquinolin-7-ol and formation of P3. 69
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(range studied: 10–500 mV s−1), pulse amplitude Epulse = 300 mV (range studied: 10–500 mV), pulse time tp = 1 ms (range studied: 1–20 ms) and scanning step Estep = 5 mV (range studied: 1–50 mV). For the 1.0 × 10−4 mol L−1 quinoline solution, the best conditions for the optimization parameters regarding SWV were: pulse amplitude Eamp = 30 mV (range studied: 1–55 mV), frequency f = 7.0 Hz (range studied: 1–100 Hz) and scanning step Estep = 10 mV (range studied: 1–20 mV).
detection limits (0.25 μg L−1 and 5,05 μg L−1 for pyridine and quinoline, respectively), but under severe reduction conditions in DMF/ TBAF4 electrolyte (−2.52 V for pyridine and −1.95 V for quinoline). 3.2.4. Determining pyridine and quinoline in fuel and textile samples Performance of the MCPE/AC electrode using DPV and SWV to determine pyridine and quinoline, respectively, was checked through analyses of samples of gasoline and textile effluents. It was verified that such samples did not present detectable quantities of analytes and, thus, curves were built with the method of standard addition. In addition, recovering trials were performed with stock solutions of pyridine and quinoline at the concentration of 1.0 × 10−2 mol L−1. When it comes to the gasoline samples, it was necessary to add surfactant to the support electrolyte to enhance sample solubilization. The surfactants were sodium dodecyl sulfate (SDS, anionic), Triton X100 (TX-100, nonionic) and cetyltrimethylammonium bromide (CTAB, cationic). Among the studied surfactants, CTAB did not interfere in the analyses of pyridine and quinoline and favored reduction, since it has NH4+ in its composition. Through voltammetry studies, it was verified that the best CTAB concentration was of 1.0 × 10−5 mol L−1. The standard addition curves and recovered studies were performed under the same experimental conditions as those of the analytical curves (Fig. 12). Fig. 12 shows the DPV voltammograms for the increasing additions of standard solutions of pyridine to textile effluent samples (A) and gasoline (B), for a concentration range of 9.0 × 10−7 to 1.0 × 10−4 mol L−1. The regression equations corresponding to the linear range of concentration for pyridine in textile effluent and gasoline samples were i = 59.40 + 1.22 × 107 c, r = 0.9904, and i = 61.87 + 7.82 × 106 c, r = 0.9901 (Fig. 12), where i is the current (in μA) and c is the pyridine concentration (in μmol L−1). By using the t-student test (confidence interval of 95%) [33] for the values of angular coefficient of the pyridine analytical curves with external standardization (8.88 × 106 μA mol−1 L) and with standard addition (1.22 × 107 μA mol−1 L and 7.82 × 106 μA mol−1 L), no significant differences were observed regarding the analyses of pyridine in textile effluent and gasoline samples. These satisfactory results were also obtained for quinoline in the same samples. Concentrations of 1.0, 10 and 100 mol L−1 were used for the recovered studies for both analytes. The environmental samples presented recovering intervals of 95 to 99% for pyridine and of 96 to 98% for quinoline, and intervals of 92 to 98% were observed for pyridine and quinoline in gasoline samples. These values (satisfactory, considering the low concentration and presence of relatively complex samples) show that the analytes did not undergo major interferences from other species present in the samples used in the experiments herein described.
3.2.2. Studies on the electochemical behavior of organic nitrogen compounds in the presence of NH4Cl Voltammetry studies were performed to verify the importance of the ammonium salt to the composition of the electrolyte solution during the electrochemical reduction of analytes. In order to do so, solutions of KCl, NH4Cl and (NH4)2SO4 were used at the concentration of 0.5 mol L−1, keeping the pH 2.5 0.02 mol L−1 BR buffer solution to ensure an acidic medium. A study with KCl and NH4Cl was performed and great differences regarding the values of peak cathodic currents were observed. To confirm the influence of the cation (NH4+) on the reduction process, the solution of NH4Cl was replaced by a (NH4)2SO4 solution, and no significant differences concerning the reduction peak currents were observed. Such observations can be seen in Fig. 10. The results obtained for pyridine (Fig. 10) were similar to those of quinoline and corroborate the cyclic voltammetry studies (Figs. 2 and 3), in which it was observed that the NH4+ ions are important for the reduction of organic nitrogen compounds. Moreover, when comparing the cathodic currents for pyridine reduction in solutions with NH4Cl and (NH4)2SO4, great variations in this parameter are not observed. Therefore, the replacement of Cl− ions with SO42− did not interfere in the electrochemical processes of the studied molecules. 3.2.3. Analytical curves for pyridine and quinoline After the optimization steps were carried out in 0.02 mol L−1 BR/ 0.5 mol L−1 NH4Cl buffer solution, using DPV for pyridine and SWV for quinoline, different aliquots from stock solutions of 1.0 × 10−2 mol L−1 organic nitrogen compounds were added to the electrolyte solution to determine the linear relationship between the cathodic current signals and the concentrations of analytes. With the results, it was possible to obtain analytical curves at the concentration range between 9.0 × 10−7 and 1.0 × 10−4 mol L−1 for pyridine (Fig. 11A) and quinoline (Fig. 11B). From Fig. 11, the regression equations corresponding to the linear concentration range of pyridine (i = 63.40 + 8.88 × 106 c, with r = 0.9903) and quinoline (i = 4.15 + 3.56 × 106 c, with r = 0.9943) were obtained, where i is the current (in μA), and c is the concentration of analytes in mol L−1. It was possible to perform 74 determinations of pyridine or quinoline by voltammetry for each MCPE/AC assembly. The detection limits estimated [32] were 2.01 × 10−7 mol L−1 (15.87 μg L−1) and 1.98 × 10−7 mol L−1 (25.54 μg L−1), and the quantification ones were 6.69 × 10−7 mol L−1 (52.85 μg L−1) and 6.53 × 10−7 mol L−1 (84.24 μg L−1) for pyridine and quinoline, respectively. Okumura and Stradiotto [21] have also achieved good 0
In this study, carbon paste electrodes modified with activated carbon were prepared and applied to analyze organic nitrogen Fig. 10. (A) Voltammograms with 0.02 mol L−1 BR buffer in the presence of different cations: (a) addition of NH4Cl, (b) addition of pyridine, (c) addition of KCl and pyridine, (d) addition of NH4Cl and pyridine; (B) 0.02 mol L−1 BR buffer containing different anions: (a) addition of NH4Cl, (b) addition of (NH4)2SO4 and pyridine, (c) addition of NH4Cl and pyridine. [Salts]: 0.5 mol L−1, [pyridine] = 1.0 −4 −1 × 10 mol L . MCPE/AC.
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Fig. 11. Analytical curves obtained after successive additions of pyridine (A) and quinoline (B) at the concentrations of 9.0 × 10−7, 1.0 × 10−6, 3.0 × 10−6, 6.0 × 10−6, 9.0 × 10−6, 1.0 × 10−5, 3.0 × 10−5, 6.0 × 10−5, −5 −4 −1 9.0 × 10 , 1.0 × 10 mol L . For pyridine: (DPV) ν = 50 mV s−1, Epulse = 300 mV, tp = 1 ms, Estep = 5 mV, and for quinoline: (SWV) Eamp = 30 mV, Estep = 10 mV, f = 7 Hz. MCPE/AC.
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Fig. 12. Analytical curves obtained after successive additions of pyridine at the concentrations of 9.0 × 10−7 to 1.0 × 10−4 mol L−1 to the samples: (A) textile effluents and (B) gasoline. ν = 50 mV s−1, Epulse = 30 mV, tp = 1 ms, Estep = 5 mV. MCPE/AC.
compounds in environmental samples. The modified electrode favored electrochemical reduction of analytes in an irreversible process with mass transfer through diffusion and contribution of adsorption. The voltammetry methods proposed were satisfactory when compared to the maximum concentration limits of such molecules required by the current legislation.
[9]
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Acknowledgements The authors gratefully acknowledge financial support from the Brazilian agencies FAPEMIG, CNPq and CAPES (Finance Code 001).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.04.038.
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