- Email: [email protected]
Electrochemical degradation of butyl paraben on platinum and glassy carbon electrodes
Electrochemical degradation of butyl paraben on platinum and glassy carbon electrodes Francisco E.R. Gomes, Nyccolas E. de Souza, Carlos A. Galinaro, Leandro O.R. Arriveti, Jairo B. de Assis, Germano Tremiliosi-Filho PII: DOI: Reference:
S1572-6657(16)30108-4 doi: 10.1016/j.jelechem.2016.03.016 JEAC 2546
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
14 November 2015 16 February 2016 12 March 2016
Please cite this article as: Francisco E.R. Gomes, Nyccolas E. de Souza, Carlos A. Galinaro, Leandro O.R. Arriveti, Jairo B. de Assis, Germano Tremiliosi-Filho, Electrochemical degradation of butyl paraben on platinum and glassy carbon electrodes, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.03.016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Electrochemical Degradation of Butyl Paraben on Platinum and Glassy Carbon Electrodes
PT
Francisco E. R. Gomes1,2, Nyccolas E. de Souza2*, Carlos A. Galinaro2, Leandro O. R.
RI
Arriveti2, Jairo B. de Assis2 and Germano Tremiliosi-Filho2,
SC
1 – Federal Institute of Education, Science and Technology of Mato Grosso – Campus Confresa, 78652-000, Confresa, MT, Brazil
NU
2 – University of São Paulo – São Carlos Chemistry Institute
AC CE P
TE
D
MA
CP 780, 13560-970, São Carlos, SP, Brazil
Keywords: Butyl Paraben; degradation, electrooxidation, glassy carbon, platinum. *Corresponding author: Nyccolas Emanuel de Souza, PhD e-mail address: [email protected] phone: +55 16 3373 9934
ACCEPTED MANUSCRIPT Abstract
In this work, the electro-oxidation of butyl paraben (BuP) in aqueous solutions under
PT
different experimental conditions was studied by cyclic voltammetry. For this purpose, platinum and glassy carbon electrodes were employed in different electrolytes such as KCl,
RI
H2SO4 and K4P2O7. The best electrochemical response was observed for glassy carbon
SC
electrode in the presence of K4P2O7 based on the oxidation peak at 0.7 V vs. Ag/AgCl. Consequently, the glassy carbon electrode was selected to further study the influence on
NU
prevention of electrode passivation of three different surfactants, namely sodium dodecyl sulfate (SDS), 4-octylphenol polyethoxylate (Triton X-100) and cetyltrimethylammonium
MA
chloride (CTAC). The best results were obtained with CTAC. In order to detect and quantify butyl paraben, differential pulse voltammetry measurements were performed with several paraben concentrations. As optimized in previous experiments, glassy carbon electrodes
D
were employed in K4P2O7 (0.1 M) and CTAC (40 µM). The limit of detection was found to
TE
be 0.1 µM, but precise quantification could be assured only above 1.0 µM. Real river water samples were analyzed and the typical concentration of butyl paraben was found to be
AC CE P
around 0.1 µM. However, selectivity tests are necessary to improve the reliability of this method. Electrolysis technique was employed in order to study the degradation of butyl paraben using the following optimized conditions: E = 1.5 V (vs. Ag/AgCl) on a glassy carbon electrode in K4P2O7 (0.1 M) and CTAC (40 µM) at pH = 5, 7 or 9. Our result showed that the degradation of butyl paraben was more efficient in acidic media. Analysis by HPLCMS technique confirmed C4H5O4-; C11H13O5-; C11H11O5- and C11H13O4- as the main degradation products.
2
ACCEPTED MANUSCRIPT
PT
1.Introduction
Parabens are esters of p-hydroxybenzoic acid that are widely used as chemical
RI
preservatives in food, cosmetics, and pharmaceuticals [1,2]. These preservatives are active against fungi, bacteria, and yeast. Furthermore, they are colorless, odorless, inexpensive, and
SC
stable in wide ranges of pH and temperature [3,4]. Parabens were first used in pharmaceuticals in the mid1920s [5] and, since then, the application of paraben
NU
preservatives has been expanded and diversified [6]. In 1981, parabens were already used in more than 13,200 pharmaceutical formulations [3]. A study conducted in 1995 analyzed 215
MA
cosmetic products and found that nearly all (99%) of leave-on products and most (77%) rinse-off products contained parabens [7]. In a recent survey conducted in 2013, a total of 282 food samples, representing 13 food categories, (i.e. cereals, meat, fish and seafood,
D
eggs, dairy products, bean products, fruits, vegetables, cookies, beverages, cooking oils,
TE
condiments, and others) were all found to contain parabens [8]. Unfortunately, some recent studies have shown that parabens possess estrogenic
AC CE P
activity since they can bind to estrogen receptors, causing unwanted effects in the organisms such as changes in hormone concentration. A relationship between breast cancer and application of paraben-containing products on skin is speculated since these compounds were found in breast tumors [9]. In 2005-2006, the analysis of urine samples from 2,548 people above the age of six years in the United States showed that 47% of the samples had butyl paraben [10]. Parabens were also found in human urine samples from Spain and Denmark [11,12]. Studies have also shown the presence of parabens in human plasma, serum and milk at concentrations of the order of a few nanograms per milliliter [13,14]. There are different types of parabens categorized according to the alkyl radical. This property affects the solubility and the antimicrobial activity [15]. The most commonly used are methyl paraben (MeP), ethyl paraben (EtP), propyl paraben (PrP) and butyl paraben (BuP); the last one displays the highest estrogenic activity [16]. An assay of estrogenic activity of parabens in yeast showed that butyl paraben was three times stronger than nonylphenol, a strong endocrine deregulator. Tests of butyl paraben in juvenile rainbow trout (Oncorhynchus mykiss) showed positive results for 3
ACCEPTED MANUSCRIPT inducing protein synthesis within the yolk (vitellogenin) [17]. Another study also reported that butyl paraben caused DNA damage in male sperm of rats [18]. In 2011, Denmark prohibited the use butyl paraben, its isoforms, and its salts, in products consumed by
PT
children under 3 years old [19].
Parabens have also been detected in sewage effluents, surface and ground water,
RI
rivers, and even in drinking water [20–23]. A study carried out in Portugal revealed the
SC
presence of parabens in treated water, demonstrating that treatment plants are not prepared to eliminate these endocrine disrupting chemicals [24]. Błędzka et al. [15] showed that the
NU
removal of endocrine disruptors at low concentrations in conventional sewage treatment plants is a difficult and costly process and that any modern techniques show good enough
MA
efficiency in the removal of certain substances. As alternative to conventional wastewater treatments, electrochemical methods are presented as promising techniques in view of their selectivity, efficiency, and environmental compatibility [25]. Szpyrkowicz et al. [26] applied
D
this method for the treatment of tannery wastewater using a Ti/Pt electrode. Kuramitz et al.
TE
[27,28] used a carbon fiber electrode to oxidize and remove bisphenol A and noniphenol. Vega et al. [29] studied the electrochemical behavior of phenolic estrogenic compounds at a
AC CE P
carbon nanotube-modified electrode aiming towards remediation of wastewater. Nevertheless, it is known that anodic oxidation of phenolic compounds inactivates the surface of graphite and noble metal electrodes through the formation of polymers that block the electrode surface (passivation). However, the use of surfactants as antifouling agents has been advocated in the literature to avoid this problem [30,31]. Hu et al. [32] successfully applied cetyltrimethylammonium bromide (CTAB) as a surfactant to improve the electrochemical degradation activity of estradiol, estrone, and estriol at a glassy carbon electrode. Brugnera et al. [33] used a screen-printed carbon electrode in CTAB micellar medium to detect the presence of bisphenol A. In the literature, there are no studies showing the electrochemical behavior of butyl paraben on platinum and glassy carbon electrodes. In addition, there are no studies related to the anodic oxidation of butyl paraben on glassy carbon electrode in the presence of surfactants. Quantification of paraben generally is performed by HPLC [34–36], however, an electrochemical quantification method was not found in the literature up to now. Few studies have reported the use of electrochemical techniques for parabens quantification [37] 4
ACCEPTED MANUSCRIPT despite the advantages in relation to cromatographyc methods. Eletroanalysis requires less time, easier sample preparation and detection limits comparable to other analytical methods. In fact, Gholivand et. al. [38] achieved limits of detection as low as 0.3 nM using
PT
Differential Pulse Voltammetry for propyl paraben detection.
Thus, in this work, we investigated the electrochemical behavior of butyl paraben
RI
using platinum and glassy carbon electrodes under different conditions and in the presence
SC
of surfactant, SDS, Triton X-100, and CTAC. Afterwards, a quantification via electrochemical method is suggested. In addition, we performed the electrochemical
NU
degradation of butyl paraben using optimized conditions and analyzed the degradation
MA
products using HPLC-MS.
2. Experimental
TE
D
2.1. Reagents
The following high-purity grade chemicals were used without further purification:
AC CE P
butyl p-hydroxybenzoate (butyl paraben), purity ≥ 99.0 % (Aldrich, USA), sodium chloride (NaCl), purity ≥ 99.0 % (Aldrich, USA), sulfuric acid (H2SO4), purity ≥ 99.0 % (Aldrich, USA), potassium diphosphate (K4P2O7), purity ≥ 99 % (Riedel-de Haen, Germany), hydrochloric acid (HCl), purity ≥ 99.0 % (Aldrich, USA), sodium hydroxide (NaOH), purity ≥ 99.0 % (Aldrich, USA), ethanol, purity ≥ 99.0 % (Qhemis, Brazil) and cetyltrimethylammonium chloride (CTAC), sodium dodecyl sulfate (SDS) and 4-octylphenol polyethoxylate (Triton X-100) (all from Aldrich, USA).
2.2. Apparatus and characterization techniques Electrochemical measurements were carried out using an Autolab® potenstiostat model PGSTAT 30 coupled to a microcomputer and managed by GPES 4.9 software. All electrochemical studies were performed in a conventional glass electrochemical cell with a working volume of 25 mL. A three-electrode configuration was employed, with a working electrode of glassy carbon or polycrystalline platinum foil (each electrode with geometric 5
ACCEPTED MANUSCRIPT area of 1.0 cm2). A platinum wire was used as the counter electrode and an Ag/AgCl electrode in 3 M KCl solution was used as the reference electrode, separated by a glass frit. Thus, all potentials mentioned in this work refer to Ag/AgCl/KCl(3M). The glassy carbon
PT
electrode was previously polished with alumina particles up to 0.05 µm size and washed with deionized water. The electrolyte solutions were 0.1 M KCl, H2SO4 or K4P2O7. Cyclic
RI
voltammetric profiles were collected at a scan rate of 20 mV s-1. Differential pulse
SC
voltammetry was employed for detection and quantification of butyl paraben, with step potential of 9 mV, modulation amplitude of 0.10 V and scan rate of 20 mV s -1.
NU
Chronoamperometric studies (electrolysis) were carried out at a constant potential of 1.5 V. The detection and quantification of butyl paraben in real water samples were
MA
performed without any pretreatment. The water samples were collected from Mogi-Guaçu river, located in the center of São Paulo state in Brazil. Two points of the river 12 Km away from each other were chosen: the first one at Rincão city, and the second one at Guatapará
D
city. Even though Guatapará city is smaller, the river crosses its urban area, where domestic
TE
sewage is directly discharged into the river. On the other hand, at Rincão city, the river crosses only the countryside.
AC CE P
For Differential Pulse Voltammetry measurements, the samples were used as collected, only adding the electrolyte and surfactant. For analysis by high performance liquid chromatography (HPLC), the samples were extracted and concentrated about 100 times by dispersive liquid-liquid microextraction (DLLME), with acetone as the dispersing agent and 1-octanol as the extractor solvent according to the method of Galinaro and Vieira 2014. The HPLC measurements were carried out using a Shimadzu® system equipped with a LC 20AD pump, a DGU-20A5 degasser, a CTO-20A oven kept at 30 °C, and a SPD-20A UV/visible detector. Methanol and water mixture (7:3 v/v) was used as mobile phase with a flow rate of 1 mL min-1 in a reverse phase C18 column (4.6 mm × 250 mm x 5 μm model Shim-pack CLC-ODS (M) Shimadzu, Japan). During the chronoamperometric oxidation measurements, 1.0 mL aliquots of the electrolyte were taken every 2.5 h and analyzed using a V-630 UV/Vis spectrophotometer (Jasco Co., Japan) equipped with a quartz cell of 1 cm pathlength in the spectral range 200400 nm. The pH solution of the electrolyte was adjusted with HCl and NaOH both at 1 M. The identification of reaction products in these sample aliquots was carried out with a liquid 6
ACCEPTED MANUSCRIPT chromatograph LC−ESI-MS (liquid chromatograph electrons spray ionization mass spectrograph) coupled with a LTQ-Orbitrap Velos Thermo Fisher Scientific mass spectrometry system (Bremen, Germany) operating in the negative ion detection mode. A
PT
reverse phase column C18 (4.6 mm × 250 mm x 5 μm), model Shim-pack CLC-ODS (M) (Shimadzu, Japan) was used with a mixture of solvents A+B (A = water/formic acid,
RI
99.9/0.01 (v/v %) and B = methanol/formic acid, 99.9/0.01 (v/v %)) as the mobile phase. A
SC
flow rate of 1.0 mL min−1 was used with the following linear eluting gradient: 0−10 min, 65 % B in A; 10−15 min, 100 % B; 15−22 min, 65 % B in A. All measurements were carried
NU
out at room temperature (25 ± 1 ºC).
Analysis of the byproducts was performed by comparing the chromatogram of the
MA
electrolyte solution prior to the application of 1.5 V as a control sample with the chromatograms of aliquots that were taken after every 2.5 h of electro-oxidation. Thus,
AC CE P
3. Results and Discussion
TE
D
besides the control sample, four other samples were identified.
3.1. The Voltammetric Behavior of Butyl Paraben at Platinum and Glassy Carbon Electrodes
Cyclic voltammograms in the absence and presence of 200 µM butyl paraben on Pt electrode are shown in Fig 1. Although different electrolytes were investigated (0.1 M KCl, H2SO4 and K4P2O7), only the results in H2SO4 electrolyte are shown. In this electrolyte, the oxidation peak for butyl paraben was the most intense and clear. Similar measurements on glassy carbon electrode are shown in Fig. 2. The best performance was observed in K4P2O7 electrolyte, with butyl paraben oxidation peak at around 0.7 V. As compared to platinum, the current densities for butyl paraben oxidation on glassy carbon were significantly lower. Other studies report butyl paraben oxidation potentials between 0.9 and 1.3 V on borondoped diamond (in phosphate/Britton-Robinson buffer of different pH) and glassy carbon (in a mixture of water, methanol and phosphoric acid) electrodes, respectively [40,41].
7
ACCEPTED MANUSCRIPT In order to evaluate the stability of the electrodes towards butyl paraben oxidation, successive cyclic voltammograms were obtained up to 5 cycles (0.0 – 1.6 V for platinum electrode, and 0.0 – 1.2 V for glassy carbon electrode). These results are shown in Fig. 1c
PT
and Fig. 2c. It was observed that, for both electrodes, the current density corresponding to the anodic oxidation of butyl paraben decreases after each voltammetric cycle and the peak
RI
potential value also shifts to more positive values. The most significant decrease of the peak
SC
current density was observed for the platinum electrode; it practically disappears after 5 voltammetric cycles (see Fig. 1c). In contrast, on the glassy carbon electrode, the peak
NU
current is lower but the peak is still observed after 5 voltammetric cycles (see Fig. 2c). This behavior can be assigned to strong adsorption of organic molecules on Pt electrode.
MA
Sometimes, these species can poison the electrode surface, compromising the catalytic activity.
Figure 1. Figure 2.
D
The activity loss on both electrodes during butyl paraben oxidation are due to the
TE
passivation of the working electrodes via formation of a film (fouling); in the case of the platinum electrode, this film could be observed with the naked eye after the voltammetric
AC CE P
experiments. The film adsorbs strongly on the electrode surface, blocking the access of the electroactive species. It is well known that the oxidation of phenolic compounds through a one or two electron process causes the inactivation of graphite or noble metal electrodes. This occurs because of the production of an unstable phenoxyl radical that undergoes dimerization and/or polymerization that results in the poisoning of the electrode surface via deposition of the electropolymerized film [42,43]. In the case of parabens, the anodic oxidation occurs with the exchange of one electron on the OH group at position four of the aromatic ring leading to the formation of a phenoxy radical [44]. Similar behavior has been observed for the electro-oxidation of parabens and other phenolic compounds in hydroalcoholic and acetic acid–acetonitrile solutions [30,43–45]. The use of surfactants as antifouling agents has been described in the literature as a successful strategy. Surfactants act adsorbing on the electrode surface via hydrophobic interactions and form a charged surfactant film with hydrophilic chains orientated towards the bulk solution. They can interact with the target compounds and induce their preconcentration in the vicinity of the electrode surface without direct contact, thus, avoiding passivation [32,33]. For these 8
ACCEPTED MANUSCRIPT reasons, the use of surfactants as an additive to the electrolyte was also investigated in this study.
PT
3.2. The Voltammetric Behavior of Butyl Paraben Using a Glassy Carbon Electrode in the
RI
Presence of Surfactant
SC
The cyclic voltammograms of butyl paraben (200 µM in 0.1 M K4P2O7) in presence of different surfactants on glassy carbon electrode are presented in Fig. 3. Three types of
NU
surfactants were used: (i) cationic (CTAC), (ii) anionic (SDS), and (iii) nonionic (Triton X100).
MA
Figure 3.
Blank tests performed in the absence of butyl paraben demonstrated that the surfactants alone showed no voltammetric response, suggesting that the observed oxidation
D
peak in presence of surfactant is only due to butyl paraben oxidation (see for example Fig.
TE
3a). The voltammograms in presence of different surfactants demonstrate that the oxidation peak potential of the butyl paraben remains unaffected by the presence of these surfactants.
AC CE P
The shape of the voltammograms is very similar with small differences in the current densities for each type of additive. The sharpest peak is observed in the presence of the CTAC surfactant.
The increase in the oxidation peak current in presence of surfactant (compare Figs. 2b and 3b) is associated with the increase in solubility of butyl paraben due to formation of micelles structures and the enhancement of the mass transport to the electrode. The term micelle refers to aggregates of surfactants that possess regions of hydrophilic and hydrophobic character. In the micelle, the hydrophilic region is aligned for contact with the aqueous phase forming a polar surface. The hydrophobic region is aligned in the reverse sense, forming a non-polar center. This feature allows increase in the solubility and diffusion of butyl paraben toward the surface of the electrode, consequently increasing the peak current. Recent studies have shown that solubility and diffusion are the main processes that control the anodic oxidation of parabens [44]. Although the paraben oxidation enhancement provided by surfactant addition culminates with the solubility and diffusion control hypothesis, further experiments are necessary to verify a diffusion controlled reaction (i.e. 9
ACCEPTED MANUSCRIPT relation between peak current with square root of the scan rate and rotating disk experiments). The results presented herein demonstrate that the anodic oxidation peak is a little
PT
more intense in the presence of CTAC than in the presence of SDS and Triton X-100. As CTAC is a cationic surfactant, it can adsorb at the solid-liquid interface by means of a
RI
hydrophobic interaction. In this process, it produces a cationic film interacting with butyl
SC
paraben via electrostatic interactions with the hydroxyl group. This increases the concentration of paraben at the electrode surface without direct contact with the electrode
NU
and consequently improves the rate of electron transfer. This interaction does not occur with the other surfactants because their hydrophobic and electrostatic interactions are different.
MA
The concentration of butyl paraben at the electrode surface with and without surfactant was calculated using the following equation:
D
Qads = nFA Γ
TE
where Qads is the charge corresponding to the anodic oxidation peak of butyl paraben (calculated directly from the area of the oxidation peak), n is the number of electrons
AC CE P
involved in the process (n = 1 for the oxidation of paraben, Radovan et al., 2008), F is the Faraday constant, A is the area of the electrode and Γ is the concentration of butyl paraben at the electrode surface. In the absence of surfactant, Qads is 413.3 µC and the calculated Γ value is 4.28 x 10-3 mol cm-2. In the presence of surfactants, the values of Qads change to 572.4, 588.5, and 640.0 µC for Triton 100X, SDS, and CTAC, respectively.
The
corresponding values of Γ were 5.93 x 10-3, 6.10 x 10-3 and 6.63 x 10-3 mol cm-2. These results suggest that the presence of surfactant leads to an increase in the surface concentration of butyl paraben which is more notable in case of CTAC surfactant. Besides increasing the solubility and diffusion of the paraben molecules, the surfactant also improves the system by preventing passivation of the electrode that is a recurring problem with the oxidation of phenolic compounds using glassy carbon, platinum, and BDD electrodes [45,46]. In the case of the cationic CTAC surfactant, the butyl paraben is inserted into the arrangement of surfactant molecules (micelle) and thus the oxidized products are incorporated into molecules of CTAC which facilitates their removal from the
10
ACCEPTED MANUSCRIPT electrode vicinity and prevents polymerization. Thus, the CTAC surfactant was chosen for the long electrolysis studies, after the optimization of its concentration.
PT
3.3. Optimization of the CTAC Surfactant Concentration for Butyl Paraben Electro-
RI
Oxidation
SC
The effect of surfactant concentration on the oxidation of butyl paraben was studied by cyclic voltammetry. The intensity of the oxidation peak was evaluated for different
NU
CTAC concentrations in the range of 0.0 to 100 µM. The results are shown in Fig. 4. It was observed that the optimum CTAC concentration is in 40 µM, indicated by the maximum
MA
observed in Fig. 4. As butyl paraben solubility in water is low (0.77 µM at 25˚C [47]), its availability near the electrode surface depends on the micelle formation. At lower surfactant concentrations, the micelle formation is limited which decreases the butyl paraben
D
availability. At higher surfactant concentrations, there is a high concentration of micelles
TE
and these micelles can interact with each other forming a unidirectional layer that blocks the interaction between the electrode and butyl paraben molecules. Hence an intermediate
AC CE P
CTAC concentration of 40 µM was selected for electrolysis experiments.
Figure 4.
3.4. Detection and Quantification of Butyl Paraben
Once the conditions for butyl paraben oxidation were optimized, differential pulse voltammetry was employed in order to detect and quantify butyl paraben in water samples. This technique was used instead of cyclic voltammetry because it of its high sensitivity, especially at low concentration of electroactive species [38]. Differential pulse voltammetry experiments were carried out on glassy carbon electrode in 0.1 M K4P2O7 + 40 µM CTAC. The concentration of butyl paraben was in the range of 0.1 µM to 1000 µM. The results are shown in Fig. 5. A concentration of 0.1 µM (ca. 30 ppb) was found to be enough for butyl paraben detection. However, reliable
11
ACCEPTED MANUSCRIPT determination of butyl paraben is better ensured at concentration above 1 µM (based on calibration curve, Fig. 5b). Among the water samples collected from the Mogi-Guaçu river, only the sample
PT
from Guatapará city exhibited a butyl paraben oxidation peak. Nevertheless, the oxidation charge was below the safe quantification range. Thus, its concentration must be lower than 1
RI
µM. The water sample collected in the Rincão city does not contain detectable quantities (by
SC
this method) of butyl paraben.
To confirm the method of detection and quantification described above, HPLC
NU
measurements were performed with the same water samples. The peak for butyl paraben was found at a retention time of 9.0 min and a calibration covering the concentration range of 50
MA
to 500 ppb was prepared. For water samples collected in the Guatapará city, the concentration of butyl paraben was estimated to be 34 ppb, whereas Rincão city water sample exhibited no detectable butyl paraben. Despite the observation that HPLC seems to
D
be more sensitive towards the detection of low paraben concentrations, it is important to
TE
note that the sample was concentrated 100 times before injection, whereas in the case of electrochemical quantification, the samples were used as collected. Thus, the
AC CE P
electrochemical method is a promising alternative to HPLC analysis. Meanwhile, the interference of other molecules (such as other parabens) must be studied to validate the butyl paraben detection by DPV. In the specific samples analyzed in this work, the DPV was verified by HPLC. However, it is impossible to predict if other molecules, by that time absent in those water samples, could result in a false positive.
Figure 5.
3.5. Electrolysis of Butyl Paraben and Identification of Byproducts
The electrolysis of butyl paraben was carried out on glassy carbon electrode in 0.1 M K4P2O7 + 40 µM CTAC at 1.5 V. This specific potential was selected because it corresponds to the region of water decomposition and the onset of oxygen evolution. Active species such as •OH, •O, and H2O2 are electro-generated from water oxidation, which can assist in oxidation of butyl paraben [48,49]. In addition to the formation of active intermediates, 12
ACCEPTED MANUSCRIPT working in this potential is beneficial because the fouling polymeric film can be mechanically removed from underneath through the evolution of oxygen bubbles [48]. Both
promoting direct and indirect oxidation of butyl paraben.
PT
effects allied with the presence of surfactant keep the initial activity of the electrode,
The butyl paraben electrolysis was carried out under acidic, neutral and basic pH
RI
conditions (pH = 5, 7 and 9, respectively) in order to evaluate the effect of pH on the
SC
degradation of the target molecule. In these conditions the degradation of the butyl paraben was more efficient in acidic media than in neutral and basic media. In order to monitor the
NU
concentration of butyl paraben throughout the electro-oxidation process, samples were analyzed by UV / visible spectroscopy. Fig 6 compares the absorbance change as a function
MA
of electro-oxidation time under different pH conditions. The absorbance decreased with time in each case, indicating electrochemical degradation of paraben, with the degradation efficiency being higher at lower pH. The observed pH dependence can be explained in terms
D
of micelle structure. At high pH values, CTAC micelles lead to the replacement of Cl- ions
TE
in the Stern layer by OH- ions. It increases the surface charge of the micelles and hinders the incorporation of butyl paraben, thus, decreasing their availability and consequently their
AC CE P
propensity for oxidation.
Figure 6.
In order to assure that real samples can be successfully treated electrochemically, the water sample collected from Guatapará city was electrolyzed and HPLC analysis was performed before and after the electrolysis at pH=7 (Fig. 7). After 2.5 hours of electrolysis, about 95% of butyl paraben was degraded, indicating that the technique is very effective for the treatment of wastewater contaminated with parabens in low concentrations.
Figure 7.
In order to evaluate the distribution of electrolysis products, a sample of the remaining electrolysis solution was analyzed by HPLC coupled to mass spectroscopy (MS). These complimentary techniques are powerful because they allow the determination of products via their mass to charge ratio (m/z) and interaction with the HPLC column 13
ACCEPTED MANUSCRIPT (retention time). Identification of the byproducts was carried out either based on comparison with database spectra or by comparing the mass spectra with literature data. The mass spectrum of the butyl paraben shows a molecular ion peak at m/z of
PT
193.087 with retention time of 11.30 minutes. The m/z ratios of the main byproducts are: 117.019 (C4H5O4-); 225.076 (C11H13O5-); 223.061 (C11H11O5-); and 209. 081 (C11H13O4-)
RI
with retention time of 2.79, 5.03, 5.11 and 7.26 minutes, respectively. According to Tay el
SC
al. (2010) and Lin et al. (2011), those compounds are succinic acid (2.79 min), dihydroxybutyl paraben (5.03 min), 4-(4-hydroxybenzoyloxy) butanoic acid or 1-hydroxy-2-oxobutyl
AC CE P
TE
D
MA
NU
(5.11 min) and monohydroxy-butyl paraben or 1-hydroxy-butylparaben (7.26 min).
14
ACCEPTED MANUSCRIPT
4. Conclusions
PT
Optimal conditions for butyl paraben electro-oxidation using glassy carbon electrode
RI
were found to be 0.1 M K4P2O7 electrolyte and 40 µM CTAC additive. In the presence of surfactant, the passivation of the working electrode was significantly suppressed. By using
SC
Differential Pulse Voltammetry (DPV), the detection of butyl paraben could be performed at 0.1 µM concentration, however, precise quantification can be assured only at a concentration
NU
of 1 µM or above. These results shows that DPV is feasible for butyl paraben detection, but selectivity tests are still needed to prove its reliability. The degradation of butyl paraben by
MA
electrolysis at 1.5 V was more efficient in acidic solution where the surfactant does not suffer from the effect of counter ions. The main degradation products are C4H5O4-; C11H13O5-; C11H11O5-; and C11H13O4-.as determined by HPLC-MS. Based on the results
D
presented herein, it is concluded that the degradation and detection of butyl paraben can be
TE
carried out effectively using a glassy carbon electrode in the presence of the cationic surfactant CTAC and the electrolysis technique can be applied to the treatment of
AC CE P
wastewater contaminated with low concentrations of parabens.
Acknowledgments
The authors gratefully acknowledge financial support from the CAPES, FAPESP and CNPq, Brazil.
15
ACCEPTED MANUSCRIPT
References F.H. Baalbaki B, Blanchin M-D, Anal Chim Acta 463 (2002) 15
[2]
B.G. Soni MG, Taylor SL, Greenberg NA, Food Chem Toxicol 40 (2002) 1335.
[3]
R.L. Elder, Int. J. Toxicol. 3 (1984) 147.
[4]
D.N. Maddox, Cosm. Toillet. 97 (1982) 85.
SC
RI
PT
[1]
T. Sabalitschka, Arch. Pharm. 268 (1930) 653.
[6]
E.J. Routledge, J. Parker, J. Odum, J. Ashby, J.P. Sumpter, Toxicol. Appl. Pharmacol. 153
NU
[5]
(1998) 12.
J.W. (1995). Rastogi, S. C., Schouten, A., De Kruijf, N., and Weijland, Contact Dermatitis 32
MA
[7]
(1995) 28.
C. Liao, F. Liu, K. Kannan, Environ. Sci. Technol. 47 (2013) 3918.
[9]
P.D. Darbre, a Aljarrah, W.R. Miller, N.G. Coldham, M.J. Sauer, G.S. Pope, J. Appl. Toxicol.
[10]
AC CE P
24 (2004) 5.
TE
D
[8]
X. Ye, A.M. Bishop, J. a. Reidy, L.L. Needham, A.M. Calafat, Environ. Health Perspect. 114 (2006) 1843.
[11]
A.A. Frederiksen H, Jørgensen N, J. Expo. Sci. Environ. Epidemiol. 21 (2011) 262.
[12]
A.M. Casas Ferreira, M. Möder, M.E. Fernández Laespada, Anal. Bioanal. Chem. 399 (2011) 945.
[13]
W.L. Margret Schlumpfa, Karin Kypkeb, Matthias Wittassekc, Juergen Angererc, Hermann Mascherd, Daniel Mascherd, Cora Vökte, Monika Birchlere, Chemosphere 81 (2010) 1171.
[14]
C.A. Ye X, Tao LJ, Needham LL, Talanta 76 (2008) 865.
[15]
D. Błędzka, D. Gryglik, J.S. Miller, J. Photochem. Photobiol. A Chem. 203 (2009) 131.
[16]
R. Golden, J. Gandy, G. Vollmer, Crit. Rev. Toxicol. 35 (2005) 435.
[17]
P.B. Bo Alslev, Bodil Korsgaard, Aquat. Toxicol. 72 (2005) 295.
16
ACCEPTED MANUSCRIPT [18]
S. Oishi, Toxicol Ind Heal. 17 (2001) 31.
[19]
S. Scientific Committee on Consumer Safety, Clarification on Opinion SCCS/1348/10 in the Light of the Danish Clause of Safeguard Banning the Use of Parabens in Cosmetic Products
PT
Intended for Children under Three Years of Age, 10 October, 2011.
S.D.C. A.J. Watkinsona, , E.J. Murbyd, D.W. Kolpine, Sci. Total Environ. 407 (2009) 2711.
[21]
H.T. Norihide Nakada, Toshikatsu Tanishima, Hiroyuki Shinohara, Kentaro Kiri, Water Res.
SC
RI
[20]
40 (2006) 3297.
M.D. Santiago Esplugasa, Daniele M. Bilab, Luiz Gustavo T. Krausec, J. Hazard. Mater. 149
NU
[22]
(2007) 631.
Zaharie Moldovan, Chemosphere 64 (2006) 1808.
[24]
L. AM, Estudo de Desreguladores Endócrinos Numa Estacão de Tratamento de Águas
MA
[23]
TE
D
Residuais (Master Thesis), University of Trás-os-Montes and Alto Douro, 2006. C. a Martínez-Huitle, S. Ferro, Chem. Soc. Rev. 35 (2006) 1324.
[26]
L. Szpyrkowicz, J. Naumczyk, F. Zilio-Grandi, Water Res. 29 (1995) 517.
[27]
H. Kuramitz, Y. Nakata, M. Kawasaki, S. Tanaka, Chemosphere 45 (2001) 37.
[28]
H. Kuramitz, J. Saitoh, T. Hattori, S. Tanaka, Water Res. 36 (2002) 3323.
[29]
D. Vega, L. Agüí, a González-Cortés, P. Yáñez-Sedeño, J.M. Pingarrón, Talanta 71 (2007)
AC CE P
[25]
1031. [30]
M.G. and D.W. KIRK, Chem. Appl. 68 (1990).
[31]
R. Vittal, H. Gomathi, K.-J. Kim, Adv. Colloid Interface Sci. 119 (2006) 55.
[32]
S. Hu, K. Wu, H. Yi, D. Cui, Anal. Chim. Acta 464 (2002) 209.
[33]
M.F. Brugnera, M.A.G. Trindade, M.V.B. Zanoni, Anal. Lett. 43 (2010) 2823.
[34]
J. I. Cacho, N. Campillo, P. Viñas, M.Hernández-Cordóba. Talanta.. 146 (2016) 568.
17
ACCEPTED MANUSCRIPT [35]
V. C. Jardim, L. P. Melo; D. S. Domingues, M. E. C. Queiroz. J. Chrom. B. 974 (2015) 35.
[36]
J-X. ZHANG, M-M. MIAO, X-M. NIU, J-Z. LI, Y. HAN, K-Q. ZHANG, C-M. Zhang. A. J.
Chem. 26 (2014) 5082.
PT
[38]
K. M. Naik, S.T. Nandibewoor. Sensors and Actuators A: Physical. 212 (2014) 127.
M.B. Gholivand, M. Shamsipur, S. Dehdashtian, H.R. Rajabi, Mater. Sci. Eng. C. Mater. Biol.
RI
[37]
C. A. Galinaro, F. M. Pereira, E. M. Vieira. J. Braz. Chem. Soc. 26 (2015) 2205.
NU
[39]
SC
Appl. 36 (2014) 102.
S.H. Kang, H. Kim, J. Pharm. Biomed. Anal. 15 (1997) 1359.
[41]
I. Martins, F.C. Carreira, L.S. Canaes, F.A. de Souza Campos Junior, L.M. da Silva Cruz, S.
MA
[40]
Rath, Talanta 85 (2011) 1.
G.P. Comninellis, Ch. and Vercesi, J. Appl. Electrochem 21 (1991) 335.
[43]
M. Gattrell and D. W. Kirk, J. Electrochem. Soc. 140 (1993) 1534.
[44]
S. Michalkiewicz, J. Appl. Electrochem. 43 (2012) 85.
[45]
C. Radovan, D. Cinghită, F. Manea, M. Mincea, C. Cofan, V. Ostafe, Sensors 8 (2008) 4330.
[46]
Y.-L. Hu, Y. Lu, G.-J. Zhou, X.-H. Xia, Talanta 74 (2008) 760.
[47]
D. Błedzka, D. Gryglik, J.S. Miller, J. Photochem. Photobiol. A Chem. 203 (2009) 131.
[48]
N. Belhadj Tahar, A. Savall, Electrochim. Acta 55 (2009) 465.
[49]
C. Feng, N. Sugiura, S. Shimada, T. Maekawa, J. Hazard. Mater. 103 (2003) 65.
AC CE P
TE
D
[42]
18
ACCEPTED MANUSCRIPT Figure Captions Figure 1. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M H2SO4) on Pt: (a) paraben and (c) fifth cycle in presence of butyl paraben.
PT
cyclic voltammogram in absence of butyl paraben, (b) first cycle in presence of butyl
Figure 2. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M K4P2O7) on glassy
RI
carbon: (a) cyclic voltammogram in absence of butyl paraben (b) first cycle in presence of
SC
butyl paraben (c) fifth cycle in presence of butyl paraben.
Figure 3. Cyclic voltammograms in absence and presence of butyl paraben (200 µM in 0.1
NU
M K4P2O7 ) and in presence of different surfactants on glassy carbon: (a) CTAC in absence of butyl paraben, (b) CTAC and butyl paraben, (c) SDS and butyl paraben and (d) TRITON
MA
X-100 and Butyl Paraben.
Figure 4. Oxidation peak current for butyl paraben (200 µM) as function of CTAC concentration.
D
Figure 5. (a) Differential pulse voltammetry in 0.1 M K4P2O7 + 40 µM CTAC with different
TE
butyl paraben concentrations glassy carbon electrode. Step potential: 9 mV, modulation amplitude: 0.10 V, scan rate: 20 mV s-1. (b) Calibration curve obtained by integration of
AC CE P
oxidation peaks.
Figure 6. Absorbance-time curves of electrolyzed solutions of butyl paraben (200 µM + 0.1 M K4P2O7 + 40 µM CTAC) measured under different pH conditions. Figure 7. Chromatograms before (a) and after (b) electrolysis of Mogi-Guaçu river water sample from Guatapará city. The electrolysis was carried out at 1.5 V in presence of 0.1 M K4P2O7 and 40 µM CTAC on Glassy Carbon.
19
ACCEPTED MANUSCRIPT
PT
Figure 1
RI
(a)
(b)
SC
0,8 0,4
NU
I / mA
1,2
MA
0,0
(c)
0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 Figure 1. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M H2SO4) on Pt: (a)
D
E / V (vs. Ag/AgCl)
and
(c)
fifth
cycle
in
presence
of
butyl
paraben.
AC CE P
paraben
TE
cyclic voltammogram in absence of butyl paraben, (b) first cycle in presence of butyl
20
ACCEPTED MANUSCRIPT
PT
Figure 2
RI
60 40
SC
I / µA
80
0 0,4
0,6
0,8
(b) (c)
1,0
1,2
MA
0,2
NU
20
(a)
E / V (vs. Ag/AgCl)
Figure 2. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M K4P2O7) on glassy
D
carbon: (a) cyclic voltammogram in absence of butyl paraben (b) first cycle in presence of
AC CE P
TE
butyl paraben (c) fifth cycle in presence of butyl paraben.
21
PT
ACCEPTED MANUSCRIPT
Figure 3
SC
RI
120
NU
I / µA
80 40
MA
0
(b)
(a)
0.4 0.6 0.8 1.0 E / V (vs. Ag/AgCl)
1.2
TE
D
0.2
(d) (c)
Figure 3. Cyclic voltammograms in absence and presence of butyl paraben (200 µM in 0.1
AC CE P
M K4P2O7 ) and in presence of different surfactants on glassy carbon: (a) CTAC in absence of butyl paraben, (b) CTAC and butyl paraben, (c) SDS and butyl paraben and (d) TRITON X-100 and Butyl Paraben.
22
ACCEPTED MANUSCRIPT
Figure 4
PT
90 80
RI SC
60 50 40
NU
I / µA cm
-2
70
20
0
MA
30 20
40
60
80
100
D
CTAC concentration / M
AC CE P
concentration.
TE
Figure 4. Oxidation peak current for butyl paraben (200 µM) as function of CTAC
23
ACCEPTED MANUSCRIPT
Figure 5
(b)
MA
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 E / V vs Ag/AgCl/KClsat
PT
6
4
SC
3
RI
5
2 1
NU
100 uA cm-²
Oxidation Charge / mC
(a)
increasing Butyl Paraben concentration
0
0
200
400
600
800 1000
Butyl Paraben Concentraion / M
Figure 5. (a) Differential pulse voltammetry in 0.1 M K4P2O7 + 40 µM CTAC with different
D
butyl paraben concentrations glassy carbon electrode. Step potential: 9 mV, modulation
AC CE P
oxidation peaks.
TE
amplitude: 0.10 V, scan rate: 20 mV s-1. (b) Calibration curve obtained by integration of
24
ACCEPTED MANUSCRIPT
Figure 6
PT
pH 9 pH 7 pH 5
RI
0.8
SC
0.6
NU
0.4 0.2 0.0 0
MA
Absorbance (254 nm)
1.0
2
4
6
8
10
12
14
D
Time / h
TE
Figure 6. Absorbance-time curves of electrolyzed solutions of butyl paraben (200
AC CE P
µM + 0.1 M K4P2O7 + 40 µM CTAC) measured under different pH conditions.
25
ACCEPTED MANUSCRIPT
PT
Figure 7 (b)
Absorbance = 10 %
SC
RI
(a)
MA
NU
Butyl Paraben
0 2 4 6 8 10 12 14 16 18 20 22
D
Retention Time / min
0 2 4 6 8 10 12 14 16 18 20 22 Retention Time / min
TE
Figure 7. Chromatograms before (a) and after (b) electrolysis of Mogi-Guaçu river water sample from Guatapará city. The electrolysis was carried out at 1.5 V in presence of
AC CE P
0.1 M K4P2O7 and 40 µM CTAC on Glassy Carbon.
26
S1572-6657(16)30108-4 doi: 10.1016/j.jelechem.2016.03.016 JEAC 2546
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
14 November 2015 16 February 2016 12 March 2016
Please cite this article as: Francisco E.R. Gomes, Nyccolas E. de Souza, Carlos A. Galinaro, Leandro O.R. Arriveti, Jairo B. de Assis, Germano Tremiliosi-Filho, Electrochemical degradation of butyl paraben on platinum and glassy carbon electrodes, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.03.016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Electrochemical Degradation of Butyl Paraben on Platinum and Glassy Carbon Electrodes
PT
Francisco E. R. Gomes1,2, Nyccolas E. de Souza2*, Carlos A. Galinaro2, Leandro O. R.
RI
Arriveti2, Jairo B. de Assis2 and Germano Tremiliosi-Filho2,
SC
1 – Federal Institute of Education, Science and Technology of Mato Grosso – Campus Confresa, 78652-000, Confresa, MT, Brazil
NU
2 – University of São Paulo – São Carlos Chemistry Institute
AC CE P
TE
D
MA
CP 780, 13560-970, São Carlos, SP, Brazil
Keywords: Butyl Paraben; degradation, electrooxidation, glassy carbon, platinum. *Corresponding author: Nyccolas Emanuel de Souza, PhD e-mail address: [email protected] phone: +55 16 3373 9934
ACCEPTED MANUSCRIPT Abstract
In this work, the electro-oxidation of butyl paraben (BuP) in aqueous solutions under
PT
different experimental conditions was studied by cyclic voltammetry. For this purpose, platinum and glassy carbon electrodes were employed in different electrolytes such as KCl,
RI
H2SO4 and K4P2O7. The best electrochemical response was observed for glassy carbon
SC
electrode in the presence of K4P2O7 based on the oxidation peak at 0.7 V vs. Ag/AgCl. Consequently, the glassy carbon electrode was selected to further study the influence on
NU
prevention of electrode passivation of three different surfactants, namely sodium dodecyl sulfate (SDS), 4-octylphenol polyethoxylate (Triton X-100) and cetyltrimethylammonium
MA
chloride (CTAC). The best results were obtained with CTAC. In order to detect and quantify butyl paraben, differential pulse voltammetry measurements were performed with several paraben concentrations. As optimized in previous experiments, glassy carbon electrodes
D
were employed in K4P2O7 (0.1 M) and CTAC (40 µM). The limit of detection was found to
TE
be 0.1 µM, but precise quantification could be assured only above 1.0 µM. Real river water samples were analyzed and the typical concentration of butyl paraben was found to be
AC CE P
around 0.1 µM. However, selectivity tests are necessary to improve the reliability of this method. Electrolysis technique was employed in order to study the degradation of butyl paraben using the following optimized conditions: E = 1.5 V (vs. Ag/AgCl) on a glassy carbon electrode in K4P2O7 (0.1 M) and CTAC (40 µM) at pH = 5, 7 or 9. Our result showed that the degradation of butyl paraben was more efficient in acidic media. Analysis by HPLCMS technique confirmed C4H5O4-; C11H13O5-; C11H11O5- and C11H13O4- as the main degradation products.
2
ACCEPTED MANUSCRIPT
PT
1.Introduction
Parabens are esters of p-hydroxybenzoic acid that are widely used as chemical
RI
preservatives in food, cosmetics, and pharmaceuticals [1,2]. These preservatives are active against fungi, bacteria, and yeast. Furthermore, they are colorless, odorless, inexpensive, and
SC
stable in wide ranges of pH and temperature [3,4]. Parabens were first used in pharmaceuticals in the mid1920s [5] and, since then, the application of paraben
NU
preservatives has been expanded and diversified [6]. In 1981, parabens were already used in more than 13,200 pharmaceutical formulations [3]. A study conducted in 1995 analyzed 215
MA
cosmetic products and found that nearly all (99%) of leave-on products and most (77%) rinse-off products contained parabens [7]. In a recent survey conducted in 2013, a total of 282 food samples, representing 13 food categories, (i.e. cereals, meat, fish and seafood,
D
eggs, dairy products, bean products, fruits, vegetables, cookies, beverages, cooking oils,
TE
condiments, and others) were all found to contain parabens [8]. Unfortunately, some recent studies have shown that parabens possess estrogenic
AC CE P
activity since they can bind to estrogen receptors, causing unwanted effects in the organisms such as changes in hormone concentration. A relationship between breast cancer and application of paraben-containing products on skin is speculated since these compounds were found in breast tumors [9]. In 2005-2006, the analysis of urine samples from 2,548 people above the age of six years in the United States showed that 47% of the samples had butyl paraben [10]. Parabens were also found in human urine samples from Spain and Denmark [11,12]. Studies have also shown the presence of parabens in human plasma, serum and milk at concentrations of the order of a few nanograms per milliliter [13,14]. There are different types of parabens categorized according to the alkyl radical. This property affects the solubility and the antimicrobial activity [15]. The most commonly used are methyl paraben (MeP), ethyl paraben (EtP), propyl paraben (PrP) and butyl paraben (BuP); the last one displays the highest estrogenic activity [16]. An assay of estrogenic activity of parabens in yeast showed that butyl paraben was three times stronger than nonylphenol, a strong endocrine deregulator. Tests of butyl paraben in juvenile rainbow trout (Oncorhynchus mykiss) showed positive results for 3
ACCEPTED MANUSCRIPT inducing protein synthesis within the yolk (vitellogenin) [17]. Another study also reported that butyl paraben caused DNA damage in male sperm of rats [18]. In 2011, Denmark prohibited the use butyl paraben, its isoforms, and its salts, in products consumed by
PT
children under 3 years old [19].
Parabens have also been detected in sewage effluents, surface and ground water,
RI
rivers, and even in drinking water [20–23]. A study carried out in Portugal revealed the
SC
presence of parabens in treated water, demonstrating that treatment plants are not prepared to eliminate these endocrine disrupting chemicals [24]. Błędzka et al. [15] showed that the
NU
removal of endocrine disruptors at low concentrations in conventional sewage treatment plants is a difficult and costly process and that any modern techniques show good enough
MA
efficiency in the removal of certain substances. As alternative to conventional wastewater treatments, electrochemical methods are presented as promising techniques in view of their selectivity, efficiency, and environmental compatibility [25]. Szpyrkowicz et al. [26] applied
D
this method for the treatment of tannery wastewater using a Ti/Pt electrode. Kuramitz et al.
TE
[27,28] used a carbon fiber electrode to oxidize and remove bisphenol A and noniphenol. Vega et al. [29] studied the electrochemical behavior of phenolic estrogenic compounds at a
AC CE P
carbon nanotube-modified electrode aiming towards remediation of wastewater. Nevertheless, it is known that anodic oxidation of phenolic compounds inactivates the surface of graphite and noble metal electrodes through the formation of polymers that block the electrode surface (passivation). However, the use of surfactants as antifouling agents has been advocated in the literature to avoid this problem [30,31]. Hu et al. [32] successfully applied cetyltrimethylammonium bromide (CTAB) as a surfactant to improve the electrochemical degradation activity of estradiol, estrone, and estriol at a glassy carbon electrode. Brugnera et al. [33] used a screen-printed carbon electrode in CTAB micellar medium to detect the presence of bisphenol A. In the literature, there are no studies showing the electrochemical behavior of butyl paraben on platinum and glassy carbon electrodes. In addition, there are no studies related to the anodic oxidation of butyl paraben on glassy carbon electrode in the presence of surfactants. Quantification of paraben generally is performed by HPLC [34–36], however, an electrochemical quantification method was not found in the literature up to now. Few studies have reported the use of electrochemical techniques for parabens quantification [37] 4
ACCEPTED MANUSCRIPT despite the advantages in relation to cromatographyc methods. Eletroanalysis requires less time, easier sample preparation and detection limits comparable to other analytical methods. In fact, Gholivand et. al. [38] achieved limits of detection as low as 0.3 nM using
PT
Differential Pulse Voltammetry for propyl paraben detection.
Thus, in this work, we investigated the electrochemical behavior of butyl paraben
RI
using platinum and glassy carbon electrodes under different conditions and in the presence
SC
of surfactant, SDS, Triton X-100, and CTAC. Afterwards, a quantification via electrochemical method is suggested. In addition, we performed the electrochemical
NU
degradation of butyl paraben using optimized conditions and analyzed the degradation
MA
products using HPLC-MS.
2. Experimental
TE
D
2.1. Reagents
The following high-purity grade chemicals were used without further purification:
AC CE P
butyl p-hydroxybenzoate (butyl paraben), purity ≥ 99.0 % (Aldrich, USA), sodium chloride (NaCl), purity ≥ 99.0 % (Aldrich, USA), sulfuric acid (H2SO4), purity ≥ 99.0 % (Aldrich, USA), potassium diphosphate (K4P2O7), purity ≥ 99 % (Riedel-de Haen, Germany), hydrochloric acid (HCl), purity ≥ 99.0 % (Aldrich, USA), sodium hydroxide (NaOH), purity ≥ 99.0 % (Aldrich, USA), ethanol, purity ≥ 99.0 % (Qhemis, Brazil) and cetyltrimethylammonium chloride (CTAC), sodium dodecyl sulfate (SDS) and 4-octylphenol polyethoxylate (Triton X-100) (all from Aldrich, USA).
2.2. Apparatus and characterization techniques Electrochemical measurements were carried out using an Autolab® potenstiostat model PGSTAT 30 coupled to a microcomputer and managed by GPES 4.9 software. All electrochemical studies were performed in a conventional glass electrochemical cell with a working volume of 25 mL. A three-electrode configuration was employed, with a working electrode of glassy carbon or polycrystalline platinum foil (each electrode with geometric 5
ACCEPTED MANUSCRIPT area of 1.0 cm2). A platinum wire was used as the counter electrode and an Ag/AgCl electrode in 3 M KCl solution was used as the reference electrode, separated by a glass frit. Thus, all potentials mentioned in this work refer to Ag/AgCl/KCl(3M). The glassy carbon
PT
electrode was previously polished with alumina particles up to 0.05 µm size and washed with deionized water. The electrolyte solutions were 0.1 M KCl, H2SO4 or K4P2O7. Cyclic
RI
voltammetric profiles were collected at a scan rate of 20 mV s-1. Differential pulse
SC
voltammetry was employed for detection and quantification of butyl paraben, with step potential of 9 mV, modulation amplitude of 0.10 V and scan rate of 20 mV s -1.
NU
Chronoamperometric studies (electrolysis) were carried out at a constant potential of 1.5 V. The detection and quantification of butyl paraben in real water samples were
MA
performed without any pretreatment. The water samples were collected from Mogi-Guaçu river, located in the center of São Paulo state in Brazil. Two points of the river 12 Km away from each other were chosen: the first one at Rincão city, and the second one at Guatapará
D
city. Even though Guatapará city is smaller, the river crosses its urban area, where domestic
TE
sewage is directly discharged into the river. On the other hand, at Rincão city, the river crosses only the countryside.
AC CE P
For Differential Pulse Voltammetry measurements, the samples were used as collected, only adding the electrolyte and surfactant. For analysis by high performance liquid chromatography (HPLC), the samples were extracted and concentrated about 100 times by dispersive liquid-liquid microextraction (DLLME), with acetone as the dispersing agent and 1-octanol as the extractor solvent according to the method of Galinaro and Vieira 2014. The HPLC measurements were carried out using a Shimadzu® system equipped with a LC 20AD pump, a DGU-20A5 degasser, a CTO-20A oven kept at 30 °C, and a SPD-20A UV/visible detector. Methanol and water mixture (7:3 v/v) was used as mobile phase with a flow rate of 1 mL min-1 in a reverse phase C18 column (4.6 mm × 250 mm x 5 μm model Shim-pack CLC-ODS (M) Shimadzu, Japan). During the chronoamperometric oxidation measurements, 1.0 mL aliquots of the electrolyte were taken every 2.5 h and analyzed using a V-630 UV/Vis spectrophotometer (Jasco Co., Japan) equipped with a quartz cell of 1 cm pathlength in the spectral range 200400 nm. The pH solution of the electrolyte was adjusted with HCl and NaOH both at 1 M. The identification of reaction products in these sample aliquots was carried out with a liquid 6
ACCEPTED MANUSCRIPT chromatograph LC−ESI-MS (liquid chromatograph electrons spray ionization mass spectrograph) coupled with a LTQ-Orbitrap Velos Thermo Fisher Scientific mass spectrometry system (Bremen, Germany) operating in the negative ion detection mode. A
PT
reverse phase column C18 (4.6 mm × 250 mm x 5 μm), model Shim-pack CLC-ODS (M) (Shimadzu, Japan) was used with a mixture of solvents A+B (A = water/formic acid,
RI
99.9/0.01 (v/v %) and B = methanol/formic acid, 99.9/0.01 (v/v %)) as the mobile phase. A
SC
flow rate of 1.0 mL min−1 was used with the following linear eluting gradient: 0−10 min, 65 % B in A; 10−15 min, 100 % B; 15−22 min, 65 % B in A. All measurements were carried
NU
out at room temperature (25 ± 1 ºC).
Analysis of the byproducts was performed by comparing the chromatogram of the
MA
electrolyte solution prior to the application of 1.5 V as a control sample with the chromatograms of aliquots that were taken after every 2.5 h of electro-oxidation. Thus,
AC CE P
3. Results and Discussion
TE
D
besides the control sample, four other samples were identified.
3.1. The Voltammetric Behavior of Butyl Paraben at Platinum and Glassy Carbon Electrodes
Cyclic voltammograms in the absence and presence of 200 µM butyl paraben on Pt electrode are shown in Fig 1. Although different electrolytes were investigated (0.1 M KCl, H2SO4 and K4P2O7), only the results in H2SO4 electrolyte are shown. In this electrolyte, the oxidation peak for butyl paraben was the most intense and clear. Similar measurements on glassy carbon electrode are shown in Fig. 2. The best performance was observed in K4P2O7 electrolyte, with butyl paraben oxidation peak at around 0.7 V. As compared to platinum, the current densities for butyl paraben oxidation on glassy carbon were significantly lower. Other studies report butyl paraben oxidation potentials between 0.9 and 1.3 V on borondoped diamond (in phosphate/Britton-Robinson buffer of different pH) and glassy carbon (in a mixture of water, methanol and phosphoric acid) electrodes, respectively [40,41].
7
ACCEPTED MANUSCRIPT In order to evaluate the stability of the electrodes towards butyl paraben oxidation, successive cyclic voltammograms were obtained up to 5 cycles (0.0 – 1.6 V for platinum electrode, and 0.0 – 1.2 V for glassy carbon electrode). These results are shown in Fig. 1c
PT
and Fig. 2c. It was observed that, for both electrodes, the current density corresponding to the anodic oxidation of butyl paraben decreases after each voltammetric cycle and the peak
RI
potential value also shifts to more positive values. The most significant decrease of the peak
SC
current density was observed for the platinum electrode; it practically disappears after 5 voltammetric cycles (see Fig. 1c). In contrast, on the glassy carbon electrode, the peak
NU
current is lower but the peak is still observed after 5 voltammetric cycles (see Fig. 2c). This behavior can be assigned to strong adsorption of organic molecules on Pt electrode.
MA
Sometimes, these species can poison the electrode surface, compromising the catalytic activity.
Figure 1. Figure 2.
D
The activity loss on both electrodes during butyl paraben oxidation are due to the
TE
passivation of the working electrodes via formation of a film (fouling); in the case of the platinum electrode, this film could be observed with the naked eye after the voltammetric
AC CE P
experiments. The film adsorbs strongly on the electrode surface, blocking the access of the electroactive species. It is well known that the oxidation of phenolic compounds through a one or two electron process causes the inactivation of graphite or noble metal electrodes. This occurs because of the production of an unstable phenoxyl radical that undergoes dimerization and/or polymerization that results in the poisoning of the electrode surface via deposition of the electropolymerized film [42,43]. In the case of parabens, the anodic oxidation occurs with the exchange of one electron on the OH group at position four of the aromatic ring leading to the formation of a phenoxy radical [44]. Similar behavior has been observed for the electro-oxidation of parabens and other phenolic compounds in hydroalcoholic and acetic acid–acetonitrile solutions [30,43–45]. The use of surfactants as antifouling agents has been described in the literature as a successful strategy. Surfactants act adsorbing on the electrode surface via hydrophobic interactions and form a charged surfactant film with hydrophilic chains orientated towards the bulk solution. They can interact with the target compounds and induce their preconcentration in the vicinity of the electrode surface without direct contact, thus, avoiding passivation [32,33]. For these 8
ACCEPTED MANUSCRIPT reasons, the use of surfactants as an additive to the electrolyte was also investigated in this study.
PT
3.2. The Voltammetric Behavior of Butyl Paraben Using a Glassy Carbon Electrode in the
RI
Presence of Surfactant
SC
The cyclic voltammograms of butyl paraben (200 µM in 0.1 M K4P2O7) in presence of different surfactants on glassy carbon electrode are presented in Fig. 3. Three types of
NU
surfactants were used: (i) cationic (CTAC), (ii) anionic (SDS), and (iii) nonionic (Triton X100).
MA
Figure 3.
Blank tests performed in the absence of butyl paraben demonstrated that the surfactants alone showed no voltammetric response, suggesting that the observed oxidation
D
peak in presence of surfactant is only due to butyl paraben oxidation (see for example Fig.
TE
3a). The voltammograms in presence of different surfactants demonstrate that the oxidation peak potential of the butyl paraben remains unaffected by the presence of these surfactants.
AC CE P
The shape of the voltammograms is very similar with small differences in the current densities for each type of additive. The sharpest peak is observed in the presence of the CTAC surfactant.
The increase in the oxidation peak current in presence of surfactant (compare Figs. 2b and 3b) is associated with the increase in solubility of butyl paraben due to formation of micelles structures and the enhancement of the mass transport to the electrode. The term micelle refers to aggregates of surfactants that possess regions of hydrophilic and hydrophobic character. In the micelle, the hydrophilic region is aligned for contact with the aqueous phase forming a polar surface. The hydrophobic region is aligned in the reverse sense, forming a non-polar center. This feature allows increase in the solubility and diffusion of butyl paraben toward the surface of the electrode, consequently increasing the peak current. Recent studies have shown that solubility and diffusion are the main processes that control the anodic oxidation of parabens [44]. Although the paraben oxidation enhancement provided by surfactant addition culminates with the solubility and diffusion control hypothesis, further experiments are necessary to verify a diffusion controlled reaction (i.e. 9
ACCEPTED MANUSCRIPT relation between peak current with square root of the scan rate and rotating disk experiments). The results presented herein demonstrate that the anodic oxidation peak is a little
PT
more intense in the presence of CTAC than in the presence of SDS and Triton X-100. As CTAC is a cationic surfactant, it can adsorb at the solid-liquid interface by means of a
RI
hydrophobic interaction. In this process, it produces a cationic film interacting with butyl
SC
paraben via electrostatic interactions with the hydroxyl group. This increases the concentration of paraben at the electrode surface without direct contact with the electrode
NU
and consequently improves the rate of electron transfer. This interaction does not occur with the other surfactants because their hydrophobic and electrostatic interactions are different.
MA
The concentration of butyl paraben at the electrode surface with and without surfactant was calculated using the following equation:
D
Qads = nFA Γ
TE
where Qads is the charge corresponding to the anodic oxidation peak of butyl paraben (calculated directly from the area of the oxidation peak), n is the number of electrons
AC CE P
involved in the process (n = 1 for the oxidation of paraben, Radovan et al., 2008), F is the Faraday constant, A is the area of the electrode and Γ is the concentration of butyl paraben at the electrode surface. In the absence of surfactant, Qads is 413.3 µC and the calculated Γ value is 4.28 x 10-3 mol cm-2. In the presence of surfactants, the values of Qads change to 572.4, 588.5, and 640.0 µC for Triton 100X, SDS, and CTAC, respectively.
The
corresponding values of Γ were 5.93 x 10-3, 6.10 x 10-3 and 6.63 x 10-3 mol cm-2. These results suggest that the presence of surfactant leads to an increase in the surface concentration of butyl paraben which is more notable in case of CTAC surfactant. Besides increasing the solubility and diffusion of the paraben molecules, the surfactant also improves the system by preventing passivation of the electrode that is a recurring problem with the oxidation of phenolic compounds using glassy carbon, platinum, and BDD electrodes [45,46]. In the case of the cationic CTAC surfactant, the butyl paraben is inserted into the arrangement of surfactant molecules (micelle) and thus the oxidized products are incorporated into molecules of CTAC which facilitates their removal from the
10
ACCEPTED MANUSCRIPT electrode vicinity and prevents polymerization. Thus, the CTAC surfactant was chosen for the long electrolysis studies, after the optimization of its concentration.
PT
3.3. Optimization of the CTAC Surfactant Concentration for Butyl Paraben Electro-
RI
Oxidation
SC
The effect of surfactant concentration on the oxidation of butyl paraben was studied by cyclic voltammetry. The intensity of the oxidation peak was evaluated for different
NU
CTAC concentrations in the range of 0.0 to 100 µM. The results are shown in Fig. 4. It was observed that the optimum CTAC concentration is in 40 µM, indicated by the maximum
MA
observed in Fig. 4. As butyl paraben solubility in water is low (0.77 µM at 25˚C [47]), its availability near the electrode surface depends on the micelle formation. At lower surfactant concentrations, the micelle formation is limited which decreases the butyl paraben
D
availability. At higher surfactant concentrations, there is a high concentration of micelles
TE
and these micelles can interact with each other forming a unidirectional layer that blocks the interaction between the electrode and butyl paraben molecules. Hence an intermediate
AC CE P
CTAC concentration of 40 µM was selected for electrolysis experiments.
Figure 4.
3.4. Detection and Quantification of Butyl Paraben
Once the conditions for butyl paraben oxidation were optimized, differential pulse voltammetry was employed in order to detect and quantify butyl paraben in water samples. This technique was used instead of cyclic voltammetry because it of its high sensitivity, especially at low concentration of electroactive species [38]. Differential pulse voltammetry experiments were carried out on glassy carbon electrode in 0.1 M K4P2O7 + 40 µM CTAC. The concentration of butyl paraben was in the range of 0.1 µM to 1000 µM. The results are shown in Fig. 5. A concentration of 0.1 µM (ca. 30 ppb) was found to be enough for butyl paraben detection. However, reliable
11
ACCEPTED MANUSCRIPT determination of butyl paraben is better ensured at concentration above 1 µM (based on calibration curve, Fig. 5b). Among the water samples collected from the Mogi-Guaçu river, only the sample
PT
from Guatapará city exhibited a butyl paraben oxidation peak. Nevertheless, the oxidation charge was below the safe quantification range. Thus, its concentration must be lower than 1
RI
µM. The water sample collected in the Rincão city does not contain detectable quantities (by
SC
this method) of butyl paraben.
To confirm the method of detection and quantification described above, HPLC
NU
measurements were performed with the same water samples. The peak for butyl paraben was found at a retention time of 9.0 min and a calibration covering the concentration range of 50
MA
to 500 ppb was prepared. For water samples collected in the Guatapará city, the concentration of butyl paraben was estimated to be 34 ppb, whereas Rincão city water sample exhibited no detectable butyl paraben. Despite the observation that HPLC seems to
D
be more sensitive towards the detection of low paraben concentrations, it is important to
TE
note that the sample was concentrated 100 times before injection, whereas in the case of electrochemical quantification, the samples were used as collected. Thus, the
AC CE P
electrochemical method is a promising alternative to HPLC analysis. Meanwhile, the interference of other molecules (such as other parabens) must be studied to validate the butyl paraben detection by DPV. In the specific samples analyzed in this work, the DPV was verified by HPLC. However, it is impossible to predict if other molecules, by that time absent in those water samples, could result in a false positive.
Figure 5.
3.5. Electrolysis of Butyl Paraben and Identification of Byproducts
The electrolysis of butyl paraben was carried out on glassy carbon electrode in 0.1 M K4P2O7 + 40 µM CTAC at 1.5 V. This specific potential was selected because it corresponds to the region of water decomposition and the onset of oxygen evolution. Active species such as •OH, •O, and H2O2 are electro-generated from water oxidation, which can assist in oxidation of butyl paraben [48,49]. In addition to the formation of active intermediates, 12
ACCEPTED MANUSCRIPT working in this potential is beneficial because the fouling polymeric film can be mechanically removed from underneath through the evolution of oxygen bubbles [48]. Both
promoting direct and indirect oxidation of butyl paraben.
PT
effects allied with the presence of surfactant keep the initial activity of the electrode,
The butyl paraben electrolysis was carried out under acidic, neutral and basic pH
RI
conditions (pH = 5, 7 and 9, respectively) in order to evaluate the effect of pH on the
SC
degradation of the target molecule. In these conditions the degradation of the butyl paraben was more efficient in acidic media than in neutral and basic media. In order to monitor the
NU
concentration of butyl paraben throughout the electro-oxidation process, samples were analyzed by UV / visible spectroscopy. Fig 6 compares the absorbance change as a function
MA
of electro-oxidation time under different pH conditions. The absorbance decreased with time in each case, indicating electrochemical degradation of paraben, with the degradation efficiency being higher at lower pH. The observed pH dependence can be explained in terms
D
of micelle structure. At high pH values, CTAC micelles lead to the replacement of Cl- ions
TE
in the Stern layer by OH- ions. It increases the surface charge of the micelles and hinders the incorporation of butyl paraben, thus, decreasing their availability and consequently their
AC CE P
propensity for oxidation.
Figure 6.
In order to assure that real samples can be successfully treated electrochemically, the water sample collected from Guatapará city was electrolyzed and HPLC analysis was performed before and after the electrolysis at pH=7 (Fig. 7). After 2.5 hours of electrolysis, about 95% of butyl paraben was degraded, indicating that the technique is very effective for the treatment of wastewater contaminated with parabens in low concentrations.
Figure 7.
In order to evaluate the distribution of electrolysis products, a sample of the remaining electrolysis solution was analyzed by HPLC coupled to mass spectroscopy (MS). These complimentary techniques are powerful because they allow the determination of products via their mass to charge ratio (m/z) and interaction with the HPLC column 13
ACCEPTED MANUSCRIPT (retention time). Identification of the byproducts was carried out either based on comparison with database spectra or by comparing the mass spectra with literature data. The mass spectrum of the butyl paraben shows a molecular ion peak at m/z of
PT
193.087 with retention time of 11.30 minutes. The m/z ratios of the main byproducts are: 117.019 (C4H5O4-); 225.076 (C11H13O5-); 223.061 (C11H11O5-); and 209. 081 (C11H13O4-)
RI
with retention time of 2.79, 5.03, 5.11 and 7.26 minutes, respectively. According to Tay el
SC
al. (2010) and Lin et al. (2011), those compounds are succinic acid (2.79 min), dihydroxybutyl paraben (5.03 min), 4-(4-hydroxybenzoyloxy) butanoic acid or 1-hydroxy-2-oxobutyl
AC CE P
TE
D
MA
NU
(5.11 min) and monohydroxy-butyl paraben or 1-hydroxy-butylparaben (7.26 min).
14
ACCEPTED MANUSCRIPT
4. Conclusions
PT
Optimal conditions for butyl paraben electro-oxidation using glassy carbon electrode
RI
were found to be 0.1 M K4P2O7 electrolyte and 40 µM CTAC additive. In the presence of surfactant, the passivation of the working electrode was significantly suppressed. By using
SC
Differential Pulse Voltammetry (DPV), the detection of butyl paraben could be performed at 0.1 µM concentration, however, precise quantification can be assured only at a concentration
NU
of 1 µM or above. These results shows that DPV is feasible for butyl paraben detection, but selectivity tests are still needed to prove its reliability. The degradation of butyl paraben by
MA
electrolysis at 1.5 V was more efficient in acidic solution where the surfactant does not suffer from the effect of counter ions. The main degradation products are C4H5O4-; C11H13O5-; C11H11O5-; and C11H13O4-.as determined by HPLC-MS. Based on the results
D
presented herein, it is concluded that the degradation and detection of butyl paraben can be
TE
carried out effectively using a glassy carbon electrode in the presence of the cationic surfactant CTAC and the electrolysis technique can be applied to the treatment of
AC CE P
wastewater contaminated with low concentrations of parabens.
Acknowledgments
The authors gratefully acknowledge financial support from the CAPES, FAPESP and CNPq, Brazil.
15
ACCEPTED MANUSCRIPT
References F.H. Baalbaki B, Blanchin M-D, Anal Chim Acta 463 (2002) 15
[2]
B.G. Soni MG, Taylor SL, Greenberg NA, Food Chem Toxicol 40 (2002) 1335.
[3]
R.L. Elder, Int. J. Toxicol. 3 (1984) 147.
[4]
D.N. Maddox, Cosm. Toillet. 97 (1982) 85.
SC
RI
PT
[1]
T. Sabalitschka, Arch. Pharm. 268 (1930) 653.
[6]
E.J. Routledge, J. Parker, J. Odum, J. Ashby, J.P. Sumpter, Toxicol. Appl. Pharmacol. 153
NU
[5]
(1998) 12.
J.W. (1995). Rastogi, S. C., Schouten, A., De Kruijf, N., and Weijland, Contact Dermatitis 32
MA
[7]
(1995) 28.
C. Liao, F. Liu, K. Kannan, Environ. Sci. Technol. 47 (2013) 3918.
[9]
P.D. Darbre, a Aljarrah, W.R. Miller, N.G. Coldham, M.J. Sauer, G.S. Pope, J. Appl. Toxicol.
[10]
AC CE P
24 (2004) 5.
TE
D
[8]
X. Ye, A.M. Bishop, J. a. Reidy, L.L. Needham, A.M. Calafat, Environ. Health Perspect. 114 (2006) 1843.
[11]
A.A. Frederiksen H, Jørgensen N, J. Expo. Sci. Environ. Epidemiol. 21 (2011) 262.
[12]
A.M. Casas Ferreira, M. Möder, M.E. Fernández Laespada, Anal. Bioanal. Chem. 399 (2011) 945.
[13]
W.L. Margret Schlumpfa, Karin Kypkeb, Matthias Wittassekc, Juergen Angererc, Hermann Mascherd, Daniel Mascherd, Cora Vökte, Monika Birchlere, Chemosphere 81 (2010) 1171.
[14]
C.A. Ye X, Tao LJ, Needham LL, Talanta 76 (2008) 865.
[15]
D. Błędzka, D. Gryglik, J.S. Miller, J. Photochem. Photobiol. A Chem. 203 (2009) 131.
[16]
R. Golden, J. Gandy, G. Vollmer, Crit. Rev. Toxicol. 35 (2005) 435.
[17]
P.B. Bo Alslev, Bodil Korsgaard, Aquat. Toxicol. 72 (2005) 295.
16
ACCEPTED MANUSCRIPT [18]
S. Oishi, Toxicol Ind Heal. 17 (2001) 31.
[19]
S. Scientific Committee on Consumer Safety, Clarification on Opinion SCCS/1348/10 in the Light of the Danish Clause of Safeguard Banning the Use of Parabens in Cosmetic Products
PT
Intended for Children under Three Years of Age, 10 October, 2011.
S.D.C. A.J. Watkinsona, , E.J. Murbyd, D.W. Kolpine, Sci. Total Environ. 407 (2009) 2711.
[21]
H.T. Norihide Nakada, Toshikatsu Tanishima, Hiroyuki Shinohara, Kentaro Kiri, Water Res.
SC
RI
[20]
40 (2006) 3297.
M.D. Santiago Esplugasa, Daniele M. Bilab, Luiz Gustavo T. Krausec, J. Hazard. Mater. 149
NU
[22]
(2007) 631.
Zaharie Moldovan, Chemosphere 64 (2006) 1808.
[24]
L. AM, Estudo de Desreguladores Endócrinos Numa Estacão de Tratamento de Águas
MA
[23]
TE
D
Residuais (Master Thesis), University of Trás-os-Montes and Alto Douro, 2006. C. a Martínez-Huitle, S. Ferro, Chem. Soc. Rev. 35 (2006) 1324.
[26]
L. Szpyrkowicz, J. Naumczyk, F. Zilio-Grandi, Water Res. 29 (1995) 517.
[27]
H. Kuramitz, Y. Nakata, M. Kawasaki, S. Tanaka, Chemosphere 45 (2001) 37.
[28]
H. Kuramitz, J. Saitoh, T. Hattori, S. Tanaka, Water Res. 36 (2002) 3323.
[29]
D. Vega, L. Agüí, a González-Cortés, P. Yáñez-Sedeño, J.M. Pingarrón, Talanta 71 (2007)
AC CE P
[25]
1031. [30]
M.G. and D.W. KIRK, Chem. Appl. 68 (1990).
[31]
R. Vittal, H. Gomathi, K.-J. Kim, Adv. Colloid Interface Sci. 119 (2006) 55.
[32]
S. Hu, K. Wu, H. Yi, D. Cui, Anal. Chim. Acta 464 (2002) 209.
[33]
M.F. Brugnera, M.A.G. Trindade, M.V.B. Zanoni, Anal. Lett. 43 (2010) 2823.
[34]
J. I. Cacho, N. Campillo, P. Viñas, M.Hernández-Cordóba. Talanta.. 146 (2016) 568.
17
ACCEPTED MANUSCRIPT [35]
V. C. Jardim, L. P. Melo; D. S. Domingues, M. E. C. Queiroz. J. Chrom. B. 974 (2015) 35.
[36]
J-X. ZHANG, M-M. MIAO, X-M. NIU, J-Z. LI, Y. HAN, K-Q. ZHANG, C-M. Zhang. A. J.
Chem. 26 (2014) 5082.
PT
[38]
K. M. Naik, S.T. Nandibewoor. Sensors and Actuators A: Physical. 212 (2014) 127.
M.B. Gholivand, M. Shamsipur, S. Dehdashtian, H.R. Rajabi, Mater. Sci. Eng. C. Mater. Biol.
RI
[37]
C. A. Galinaro, F. M. Pereira, E. M. Vieira. J. Braz. Chem. Soc. 26 (2015) 2205.
NU
[39]
SC
Appl. 36 (2014) 102.
S.H. Kang, H. Kim, J. Pharm. Biomed. Anal. 15 (1997) 1359.
[41]
I. Martins, F.C. Carreira, L.S. Canaes, F.A. de Souza Campos Junior, L.M. da Silva Cruz, S.
MA
[40]
Rath, Talanta 85 (2011) 1.
G.P. Comninellis, Ch. and Vercesi, J. Appl. Electrochem 21 (1991) 335.
[43]
M. Gattrell and D. W. Kirk, J. Electrochem. Soc. 140 (1993) 1534.
[44]
S. Michalkiewicz, J. Appl. Electrochem. 43 (2012) 85.
[45]
C. Radovan, D. Cinghită, F. Manea, M. Mincea, C. Cofan, V. Ostafe, Sensors 8 (2008) 4330.
[46]
Y.-L. Hu, Y. Lu, G.-J. Zhou, X.-H. Xia, Talanta 74 (2008) 760.
[47]
D. Błedzka, D. Gryglik, J.S. Miller, J. Photochem. Photobiol. A Chem. 203 (2009) 131.
[48]
N. Belhadj Tahar, A. Savall, Electrochim. Acta 55 (2009) 465.
[49]
C. Feng, N. Sugiura, S. Shimada, T. Maekawa, J. Hazard. Mater. 103 (2003) 65.
AC CE P
TE
D
[42]
18
ACCEPTED MANUSCRIPT Figure Captions Figure 1. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M H2SO4) on Pt: (a) paraben and (c) fifth cycle in presence of butyl paraben.
PT
cyclic voltammogram in absence of butyl paraben, (b) first cycle in presence of butyl
Figure 2. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M K4P2O7) on glassy
RI
carbon: (a) cyclic voltammogram in absence of butyl paraben (b) first cycle in presence of
SC
butyl paraben (c) fifth cycle in presence of butyl paraben.
Figure 3. Cyclic voltammograms in absence and presence of butyl paraben (200 µM in 0.1
NU
M K4P2O7 ) and in presence of different surfactants on glassy carbon: (a) CTAC in absence of butyl paraben, (b) CTAC and butyl paraben, (c) SDS and butyl paraben and (d) TRITON
MA
X-100 and Butyl Paraben.
Figure 4. Oxidation peak current for butyl paraben (200 µM) as function of CTAC concentration.
D
Figure 5. (a) Differential pulse voltammetry in 0.1 M K4P2O7 + 40 µM CTAC with different
TE
butyl paraben concentrations glassy carbon electrode. Step potential: 9 mV, modulation amplitude: 0.10 V, scan rate: 20 mV s-1. (b) Calibration curve obtained by integration of
AC CE P
oxidation peaks.
Figure 6. Absorbance-time curves of electrolyzed solutions of butyl paraben (200 µM + 0.1 M K4P2O7 + 40 µM CTAC) measured under different pH conditions. Figure 7. Chromatograms before (a) and after (b) electrolysis of Mogi-Guaçu river water sample from Guatapará city. The electrolysis was carried out at 1.5 V in presence of 0.1 M K4P2O7 and 40 µM CTAC on Glassy Carbon.
19
ACCEPTED MANUSCRIPT
PT
Figure 1
RI
(a)
(b)
SC
0,8 0,4
NU
I / mA
1,2
MA
0,0
(c)
0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 Figure 1. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M H2SO4) on Pt: (a)
D
E / V (vs. Ag/AgCl)
and
(c)
fifth
cycle
in
presence
of
butyl
paraben.
AC CE P
paraben
TE
cyclic voltammogram in absence of butyl paraben, (b) first cycle in presence of butyl
20
ACCEPTED MANUSCRIPT
PT
Figure 2
RI
60 40
SC
I / µA
80
0 0,4
0,6
0,8
(b) (c)
1,0
1,2
MA
0,2
NU
20
(a)
E / V (vs. Ag/AgCl)
Figure 2. Cyclic voltammograms of butyl paraben (200 µM in 0.1 M K4P2O7) on glassy
D
carbon: (a) cyclic voltammogram in absence of butyl paraben (b) first cycle in presence of
AC CE P
TE
butyl paraben (c) fifth cycle in presence of butyl paraben.
21
PT
ACCEPTED MANUSCRIPT
Figure 3
SC
RI
120
NU
I / µA
80 40
MA
0
(b)
(a)
0.4 0.6 0.8 1.0 E / V (vs. Ag/AgCl)
1.2
TE
D
0.2
(d) (c)
Figure 3. Cyclic voltammograms in absence and presence of butyl paraben (200 µM in 0.1
AC CE P
M K4P2O7 ) and in presence of different surfactants on glassy carbon: (a) CTAC in absence of butyl paraben, (b) CTAC and butyl paraben, (c) SDS and butyl paraben and (d) TRITON X-100 and Butyl Paraben.
22
ACCEPTED MANUSCRIPT
Figure 4
PT
90 80
RI SC
60 50 40
NU
I / µA cm
-2
70
20
0
MA
30 20
40
60
80
100
D
CTAC concentration / M
AC CE P
concentration.
TE
Figure 4. Oxidation peak current for butyl paraben (200 µM) as function of CTAC
23
ACCEPTED MANUSCRIPT
Figure 5
(b)
MA
0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 E / V vs Ag/AgCl/KClsat
PT
6
4
SC
3
RI
5
2 1
NU
100 uA cm-²
Oxidation Charge / mC
(a)
increasing Butyl Paraben concentration
0
0
200
400
600
800 1000
Butyl Paraben Concentraion / M
Figure 5. (a) Differential pulse voltammetry in 0.1 M K4P2O7 + 40 µM CTAC with different
D
butyl paraben concentrations glassy carbon electrode. Step potential: 9 mV, modulation
AC CE P
oxidation peaks.
TE
amplitude: 0.10 V, scan rate: 20 mV s-1. (b) Calibration curve obtained by integration of
24
ACCEPTED MANUSCRIPT
Figure 6
PT
pH 9 pH 7 pH 5
RI
0.8
SC
0.6
NU
0.4 0.2 0.0 0
MA
Absorbance (254 nm)
1.0
2
4
6
8
10
12
14
D
Time / h
TE
Figure 6. Absorbance-time curves of electrolyzed solutions of butyl paraben (200
AC CE P
µM + 0.1 M K4P2O7 + 40 µM CTAC) measured under different pH conditions.
25
ACCEPTED MANUSCRIPT
PT
Figure 7 (b)
Absorbance = 10 %
SC
RI
(a)
MA
NU
Butyl Paraben
0 2 4 6 8 10 12 14 16 18 20 22
D
Retention Time / min
0 2 4 6 8 10 12 14 16 18 20 22 Retention Time / min
TE
Figure 7. Chromatograms before (a) and after (b) electrolysis of Mogi-Guaçu river water sample from Guatapará city. The electrolysis was carried out at 1.5 V in presence of
AC CE P
0.1 M K4P2O7 and 40 µM CTAC on Glassy Carbon.
26