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The use of activated biochar for development of a sensitive electrochemical sensor for determination of methyl parathion
Accepted Manuscript The use of activated biochar for development of a sensitive electrochemical sensor for determination of methyl parathion
Paulo Roberto de Oliveira, Cristiane Kalinke, Jeferson Luiz Gogola, Antonio Salvio Mangrich, Luiz Humberto Marcolino Junior, Márcio F. Bergamini PII: DOI: Reference:
S1572-6657(17)30448-4 doi: 10.1016/j.jelechem.2017.06.020 JEAC 3351
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
13 February 2017 15 May 2017 12 June 2017
Please cite this article as: Paulo Roberto de Oliveira, Cristiane Kalinke, Jeferson Luiz Gogola, Antonio Salvio Mangrich, Luiz Humberto Marcolino Junior, Márcio F. Bergamini , The use of activated biochar for development of a sensitive electrochemical sensor for determination of methyl parathion, Journal of Electroanalytical Chemistry (2017), doi: 10.1016/j.jelechem.2017.06.020
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ACCEPTED MANUSCRIPT The use of activated biochar for development of a sensitive electrochemical sensor for determination of methyl parathion
Paulo Roberto de Oliveiraa, Cristiane Kalinkea, Jeferson Luiz Gogolaa, Antonio Salvio
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Mangrichb,c, Luiz Humberto Marcolino Juniora, Márcio F. Bergaminia*
a- Laboratório de Sensores Eletroquímicos (LabSensE), Departamento de Química,
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Universidade Federal do Paraná (UFPR), CEP 81.531-980, Curitiba-PR, Brazil.
b- Laboratório de Química de Húmus e Fertilizantes, Departamento de Química, Universidade
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Federal do Paraná (UFPR), CEP 81.531-980, Curitiba-PR, Brazil.
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c- Instituto Nacional de Ciência e Tecnologia de Energia e Ambiente (INCT E&A/CNPq),
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Brazil.
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*email address: [email protected]
Telephone number: +55 41 3361-3177
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Fax number: +55 41 3361-3186
ACCEPTED MANUSCRIPT
Abstract Biochar is a carbonaceous material that exhibits rich surface functional groups, which makes its use as a modifier of electrodes a very attractive alternative, especially the preconcentration of species of interest, organic or inorganic. This capability can be enhanced
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by activation of the material that increases the amount of functional groups, hence the ability to preconcentrate more species. In this study we used a carbon paste electrode modified with
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biochar activated with nitric acid for spontaneous preconcentration of Methyl Parathion and
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for further quantitative determination in drinking water. The electrode showed good
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sensitivity and limits of detection of MP, 760 µA L mmol-1, 39.0 nmol L-1, respectively. Good accuracy of the method to determine MP in tap water samples was observed with recoveries
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values varying between 102 to 110 % for three different concentration levels.
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Keywords: Activated biochar, methyl parathion, electrochemical sensor, spontaneous
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preconcentration.
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1. Introduction Biochar is a material which is rich in carbon and is produced by pyrolysis of biomass at moderate temperature conditions (< 700 ºC) and low amount or absence of oxygen [1]. The characteristics of this material simulate those observed for the “Terra Preta de Índio da
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Amazônia”, which consist of an inert internal structure and a highly functionalized surface with the ability to interact with different compounds [2-4]. The addition of other species on
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biochar can promote greater surface functionalization, forming an activated material with
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unique characteristics [5]. In general, this property makes the use of biochar advantageous in
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removing organic and inorganic contaminants in soil and water matrices [6-8]. Chemical activation of biochar is a commonly used method [9, 10]. For this reason, chemical agents are
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used for modifying the carbonaceous material surface, such as nitric acid [11], sodium hydroxide [12], hydrogen peroxide [13], among others.
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Some studies have reported that the use of biochar as a modifier of electrodes can
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improve the capacity of preconcentration of metallic ions at low limits of determination [1417]. For example, Kalinke et al. [18] report the quantification of paraquat using a carbon paste
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electrode modified with biochar. However, the determination of organic compounds using electrodes modified with biochar still requires further exploration. Nevertheless, the improved
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detectability of the proposed device in comparison to unmodified electrodes has been attributed to the presence of biochar. This is due to the ability of biochar to interact with cations of paraquat molecule. The biochar surface has negative charges which combine with the positively charged paraquat cations by electrostatic interactions [19]. Thus, the proposed method presents sensitivity of 19.2 µA L µmol-1 and limit of detection of 7.5 nmol L-1. The paraquat determination in spiked samples presents recovery values of 95.8 % and 97.5 % for coconut and natural water. 1
ACCEPTED MANUSCRIPT The interaction between biochar and other pesticides, such as methyl parathion (MP), has constantly been reported in the literature [20, 21]. The sorption of this pesticide on biochar was related to combined mechanisms of π–π interaction between the aromatic molecule of MP and the surface of biochar [19, 22], as well as interactions between the MP nitrophenyl group and the functional groups of biochar by hydrogen bonds [23, 24]. Methyl
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Parathion (O,O-dimethyl O-4-nitrophenyl phosphorothioate) is an organophosphate
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insecticide [25]. MP is highly toxic by inhalation and ingestion, and moderately toxic by
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dermal absorption [26, 27]. The US-EPA classifies it as toxicity class I, the most toxic, and its use has been banned in the EU since 2005 [28]. In Brazil, MP is largely employed to combat
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pests in many crops, mainly vegetables, fruits and cereals [29]. This pesticide is usually determined by gas chromatography (GC) [30, 31], liquid chromatography (HPLC) [32, 33]
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and voltammetric analysis, mostly using differential pulse or square-wave voltammetry [3437].
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Since there are no reports of the analytical performance of electrodes modified with
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activated biochar, and MP exhibits a good adsorption in biochar, the present paper reports for the first time the use of voltammetric determination of MP using carbon paste modified
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electrode (CPME) with biochar activated with HNO3.
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2. Material and methods
2.1 Materials and Chemicals Powdered graphite and mineral oil were acquired from Merck and Fisher Scientific, respectively (carbon paste), the standard of methyl parathion from Sigma Aldrich. Analytical grade glacial acetic acid (Isofar), anhydrous sodium acetate (J. T. Baker), acetonitrile (Sigma Aldrich), hydrochloric acid (F. Maia) and sodium hydroxide (Cromato) were used as received. All solutions were prepared with deionized water obtained with a Millipore Milli-Q system. 2
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2.2 Preparation and characterization of biochar sample First, precursor biochar samples were obtained from castor oil cake biomass for pyrolysis process at temperature of 400 ºC, heating rate of 5 ºC min-1 and residence time of 60 minutes. The sample was subjected to surface treatment process by chemical activation in
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order to increase the amount of surface functional groups and improve the preconcentration
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capacity of the material. For the surface treatment, HNO3 solution 50 % (v/v) was added to 1 g of precursor biochar. The dispersion was placed in a reflux system under constant stirring in
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temperature at 60 ºC for 3 hours. After the chemical activation step, the dispersion was
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filtered, washed with distilled water, dried in an oven at 100 °C for 24 hours, and stored for
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later use as modifier agent of electrodes.
2.3 Electrode fabrication
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The carbon pastes employed in this work consisted of powdered graphite, mineral oil
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and the modifier (biochar). The proportion utilized was 25% (w/w) of mineral oil, 75–X% of graphite and X% (w/w) of biochar in the ratios of X = 5, 10, 15, 25, 30 and 50%. Pastes were
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prepared by simple mixing and homogenization of the components. The carbon paste electrode was made by compacting of carbon paste into a plastic cylinder with an internal
support.
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diameter of 3 mm and a copper wire (electrical connection) with similar thickness to plastic
2.4 Characterization Measurements The methyl parathion electrochemical characterization was performed by cyclic voltammetry employ of the carbon paste electrode modified with biochar in presence of methyl parathion (1.0 x 10-4 mol L-1). Cyclic voltammograms (n = 5) with scan rate of 50 mV 3
ACCEPTED MANUSCRIPT s-1 were obtained and the initial scan was taken from 0.4 V to -1.2 V. For the voltammetric measurements a 10.0 mL voltammetric cell with the conventional three-electrode configuration was employed, in which the working, auxiliary and reference electrodes were CPE unmodified, and CPME with activated carbon (AC) and biochar, Pt wire and Ag/AgCl in 3.0
mol
L-1
KCl
respectively.
The
results
were
performed
on
an
AutoLab
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potentiostat/galvanostat (Metrohn), with data acquisition and experimental control by the
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NOVA software version 2.0.
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2.5 Voltammetric analysis
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The analytical procedure for determination of the MP occurred in three steps: (1) Preconcentration of MP on the electrode surface (open circuit) by stirring for five minutes in
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0.1 mol L-1 acetate buffer solution (pH 5) content 1.0 x 10-4 mol L-1 of MP; (2) Voltammetric analysis of the electrode containing MP adsorbed in 0.1 mol L-1 acetate buffer solution (pH 4)
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by differential pulse cathodic stripping voltammetry (DPCSV), the range potential being
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subsequently held from 0.4 V to -1.2 V, pulse amplitude of 100 mV s-1, and pulse time modulation of 100 ms; and (3) Renewal of voltammetric surface by methyl parathion removal
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from polishing the surface.
Experimental parameters were investigated for each step involved in the
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preconcentration and determination of MP, such as preconcentration and measuring solutions, electrode construction conditions and instrumental parameters of DPV technique.
2.6 Validation of the analytical method The analytical validation of the proposed method was done for the carrying out of the analytical curve, presence of concomitant species and determination of MP in tap water spiked samples. For concomitant species studies, copper (II), iron (III) and zinc (II) ions, 4
ACCEPTED MANUSCRIPT ascorbic acid and glucose were used at three different concentrations: 1.0 x 10-6, 1.0 x 10-5 and 1.0 x 10-4 mol L-1. The species were separately added into preconcentration solution in presence of 1.0 x 10-5 mol L-1 MP. The effect of the interfering species was evaluated by comparing the response signal of MP. The determination of MP in the spiked sample was determined by the standard addition method with four successive additions. The voltammetric
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measurement was performed using the CPME and with previously optimized experimental
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conditions.
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3. Results and discussion
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3.1 Voltammetric evaluation of the CPME with biochar for Methyl Parathion preconcentration
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The evaluation of biochar as a modifier was performed through a study comparing the ability to preconcentrate methyl parathion for three different electrodes: unmodified carbon
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paste electrode (CPE), carbon paste electrode modified with activated carbon (CPE-AC) and
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carbon paste electrode modified with biochar (CPME). The three electrodes were submitted to the same process of preconcentration in pH 5.0 acetate buffer solution containing 1.0 x 10 -4
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mol L-1 MP in the absence of applied potential and constant stirring for 5 minutes.
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Voltammograms obtained with this evaluation are shown in Figure 1. FIGURE 1
Based on voltammograms in Figure 1, it is possible to affirm that the three evaluated electrodes exhibit the adsorbing capacity of the MP because of the observed peak current at 0.6 V in all electrodes studied. The ability to preconcentrate MP of the unmodified electrode may be associated with the existence of π-π interactions between the graphite and the surface of the aromatic ring present in the MP molecule [19, 20]. For the electrode modified with activated carbon, the strongest signal can be attributed to the presence of pores in the coal 5
ACCEPTED MANUSCRIPT surface (physical adsorption) and also by the existence of π-π interactions as mentioned above, since there is the presence of 50% (w/w) of graphite in the composition of this electrode. The results obtained using the electrodes modified with the biochar showed greater intensity of current compared to the unmodified electrode and the activated carbon modified
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electrode. The higher ability of the biochar towards preconcentration of methyl parathion on
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the electrode surface is related with functional groups present at biochar surface when
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compared to graphite and activated carbon. We believe that the mechanism of interaction between the MP and the surface of the electrode modified with biochar is similar to that
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proposed by Liu et al [38] concerning the adsorption of phenol by biochars that present great amounts groups that contain nitrogen or oxygen. According to the authors, the interaction
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between the organic molecule and the surface of biochar can mainly occur in three ways: the first (1) is the physical adsorption [39]; the second (2) is attributed to π-π interactions between
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the aromatic ring of the analyte and biochar aromatic structures (hydrophobic interactions)
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[39, 40], and the third and most important (dominant interaction mechanism) (3) is the chemical interaction of the organic molecules with the functional groups on the adsorbent
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material surface. In this case between nitrophenyl group of MP and carboxyl and hydroxyl groups of biochar (hydrogen bonds) [38, 41]. Thus the modified electrode with biochar can
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present different adsorption mechanisms with the MP which together can result in a combined effect improving the response signal of sensor [38, 39, 42] (Figure 2). FIGURE 2 However, it was observed that CPME with activated biochar showed better analytical performance in comparison to the precursor. This can be due to differences in physicochemical characteristics of biochar samples after chemical treatment, thus 6
ACCEPTED MANUSCRIPT necessitating morphological characterization by scanning electron microscopy (SEM), semiquantitative elemental composition by energy dispersive spectroscopy (EDS), and estimative of surface acid groups by the Boehm method.
3.1.1 Morphological characterization
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Scanning electron microscopy (SEM) analyses were performed so as to evaluate the
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morphological characteristics of biochar samples. The representative SEM images for samples
3.
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FIGURE 3
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of precursor biochar (untreated) and after chemical treatment (activated) are shown in Figure
From the SEM images, it is possible to observe some morphological variations
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between samples. Among these modifications is a small decrease of particle size with the employment of chemical treatment (Figure 3-A, C). Some works in the literature [43, 44]
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report that acid chemical treatment may cause a significant decrease of pore volume.
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Furthermore, some authors suggest that the use of long reflux times may also affect the structure of materials [45]. These morphological variations can be explained by the oxidation
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promoted by the HNO3, resulting in the formation of functional groups on the walls or edges of pore opening, as shown in Figure 3 (B, D) [46, 47]. Thus, it is possible to suggest that a
D).
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partial “obstruction” of the pores occurs after the biochar activation, as shown in Figure 3 (B,
3.1.2 Chemical composition The chemical mapping performed using an energy dispersive spectroscopy Thermo 200 coupled with SEM allows the elemental composition quantification of the samples exposed to the electron beam [48]. Surface chemical analysis by EDS was performed for 7
ACCEPTED MANUSCRIPT precursor and activated biochar samples, and from the results obtained it was possible to identify and quantify (semi-quantitatively) the elemental composition of the samples (Table 1). TABLE 1 From the EDS results, the existence of characteristic elements of biochar was observed
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in both samples [49, 50]. A material rich in carbon, with significant amounts of oxygen and
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nitrogen in its composition can be seen. Mineral compounds, such as magnesium, silicon,
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sulfur, potassium and calcium are also commonly found in the product of the pyrolysis and are derived from the feedstock. Thus, the elemental composition may be dependent on the
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biomass adopted [51].
For the activated sample, a poorer elemental composition was observed, possibly due
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to the fact that other elements may have been removed by way of the chemical treatment. From the composition of the samples it was possible to see variations consistent with other
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reports in the literature [46, 52]. Among these, one can emphasize the reduction of carbon in
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activated biochar compared to the precursor, which might have been caused by the formation of surface oxygen groups in the biochar and also for some mineralization of carbonaceous
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matrix [53]. The formation of these functional groups may occur in the aliphatic portion of the molecule by breaking the benzylic carbons of C–C bonds or by oxidation reactions involving
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methylene (–CH2) [54]. Parallel to this, nitrate ions in an acid medium are good oxidizing agents, and can promote the oxidation of the biochar surface, leading to formation of hydroxyl and carboxylic groups and increased oxygen levels. The increased nitrogen content may be related to the formation of nitro groups. The dissociation of HNO3 can generate the nitronium ions, which reacts with the aromatic rings of the biochar forming nitrated products (–NO2) superficially adsorbed [55]. In addition, this treatment can release ions from groups existing in 8
ACCEPTED MANUSCRIPT biochar, H+ ions can displace and/or solubilize cations that are present in the biochar surface. This allows functional groups of material to be available to interact with other compounds [56, 57].
3.1.3 Estimation of Functional groups
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Boehm titration [58, 59] was performed to estimate the total acid groups (carboxylic
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acid, lactonic and phenolic), in equivalent per gram of biochar (mmol g-1) present on the surface of the samples. The potentiometric titrations were performed with 0.1 mol L-1 NaOH
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as titrant and using a Metrohm pH meter 780. An increase of total acid groups from 4.98 ±
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0.29 to 7.87 ± 0.16 mmol g-1 was observed for precursor and activated biochar, respectively. The data show the existence of a greater amount of total acid groups on the surface of biochar
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after the chemical treatment performed. These results are in concordance with other reports in the literature [43]. This means that the chemical treatment with HNO3 was effective in the
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incorporation of oxygenated functional groups on the biochar.
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With the results from characterization analyses, differences among biochar samples before and after chemical treatment were detectable. FTIR spectrum (Figure S1) revealed the
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presence of different surface functional groups for activated biochar in comparison to the precursor biochar,, which is consistent with the increase of the oxygen and nitrogen
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composition. To verify the influence of porosity and surface area on the adsorptive capacity of the materials analysis by B.E.T. method were carried out. The activated biochar showed an increase of surface properties with a pore volume increase of 80 % in comparison to the precursor material (Figure S2). This means that the porosity of the activated biochar may have corroborated to the better performance of the MP preconcentration. As a result, the best response was obtained with voltammetric electrode modified with the treated biochar. This 9
ACCEPTED MANUSCRIPT demonstrates the use of this material as an electrodes modifier, in order to increase the device’s capacity of spontaneous preconcentration and the determination of species with high sensitivity and detectability.
3.2 Effect of the Experimental Parameters on the Voltammetric Response
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The ability of biochar to adsorb MP has already been well attested elsewhere in the
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literature. Thus, the study to assess the best composition of the modified carbon paste with biochar is extremely important to obtain the best voltammetric response. The evaluation was
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made using carbon pastes prepared with 0 to 50% (w/w) of biochar content under DPAdS
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voltammetry conditions in pH 5.0 acetate buffer solution containing 5.0 x 10-5 mol L-1 to MP time of 5 minutes at open circuit potential condition.
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There was a significant increase in the intensity of MP reduction current when the amount of biochar was escalated to 10% (w/w). The elevated amount of biochar leads to an
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increase in functional groups available for interaction with MP in the folder, and these are
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mainly carboxyl, hydroxyl, lactonic and phenolic groups [41, 60]. For quantities greater than 10% (w/w) the current intensity becomes practically constant, which defines this as the best
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composition of the carbon paste.
In order to obtain higher sensitivity and shorter analysis time, we evaluated the best
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contact time of CPME in the preconcentration solution with MP. In this study, different solutions of MP were used (1.0 x 10-6, 1.0 x 10-5 and 5.0 x 10-5 mol L-1), time of immersion 0.5 to 20 minutes in acetate buffer pH 5 solution (Figure 4). FIGURE 4 For the three concentrations evaluated, the same tendency of increase of current was observed, compared to the time of preconcentration. Just as the preconcentration time raises 10
ACCEPTED MANUSCRIPT the peak current intensity, it also increases until its stabilization. This tendency occurs because the greater the exposure time of CPME solution containing the MP, the more species are preconcentrated and thus the larger is the signal. The stabilization of the signal occurs in 5 minutes and may be explained by the equilibrium between the adsorption capacity of the material with the amount of MP in solution.
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The parameters of the differential pulse technique directly influence the voltammetric
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response for MP. The optimized parameters were as follows: biochar proportion of 10 %
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(m/m), preconcentration time of 5 min in pH 5 acetate buffer solution, support electrolyte of pH 4 acetate buffer solution, pulse amplitude and duration of 100 mV and 100 ms,
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respectively.
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3.3 Analytical performance
An analytical curve was made by employing differential pulse CPME with the
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previously optimized experimental conditions. To that end, voltammograms were obtained
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(n=3) with different concentrations of MP. Analytical curves were obtained for CPME with activated (●) and not activated (■) biochar samples (Figure 5B). Figure 5A shows
FIGURE 5
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representative voltammograms for CPME with activated biochar.
It is observed that CPME-AB analytical performance was higher than obtained with the CPME-PB. This information is based on the highest sensitivity obtained for the treated biochar (0.76 µA L µmol-1) compared to the untreated (0.46 µA L µmol-1), and also from the LDR, being 0.1 to 70 µmol L-1 for the CPME-AB and 0.1 to 50 µmol L-1 for the CPME-PB. This better performance can be attributed to the greater amount of functional groups present 11
ACCEPTED MANUSCRIPT on the activated material. In other words, it is possible to establish a relation between increase of functional groups of the biochar surface and the improvement in the preconcentration capacity of methyl parathion at electrode surface. Chemical treatment with HNO3 can generate some significant changes on biochar surface as the increase of oxygen and nitrogen chemical groups which promote an improvement in the sensor response.
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Analytical performance for proposed electrodes (CPME-AB and CPME-PB) shown to
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be consistent with other works reported in the literature for the determination of methyl
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parathion, with good LDR, LOD and sensitivity (Table 2). Furthermore, most of the other methodologies reported to the MP determination in the measure solution (support electrolyte)
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[24, 61, 62]. On the other hand, in this work the preconcentration step is spontaneous, without electric potential application. This can minimize assay interference of concomitant species
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and can also enable the development of a passive sampler of methyl parathion. In addition, it should be emphasized that the proposed method presents a greater simplicity and lower cost
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in the construction of the electrodes compared to other works [63, 64]. TABLE 2
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3.4 Effect of concomitant species and applicability of the proposed sensor The results qualify the proposed method for the determination of MP in real samples,
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such as drinking water and fruit juices. It is for this reason that the study was conducted in the presence of concomitant species. Copper (II), iron (III) and zinc (II) ions were evaluated, as well as ascorbic acid and glucose. For this study, voltammetric measurements were performed in the absence and presence of these species in the preconcentration solution. The concentration of MP used in all the measures was set at 1.0 x 10-5 mol L-1, and for the assessed concomitant species three different concentrations were used: 1.0 x 10-6; 1.0 x 10-5 12
ACCEPTED MANUSCRIPT and 1.0 x 10-4 mol L-1. Significant interference was observed for the measures carried out in the presence of iron (III) ions and ascorbic acid in concentrations above 1.0 x 10-5 mol L-1; due to competition for adsorptive sites biochar among these species and MP. In this case, it would require a pretreatment of samples containing these species before the quantification MP.
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In this study, the determination in spiked samples of drinking water was executed
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without any prior treatment at three different concentrations of MP: 1.0 x 10 -6; 5.0 x 10-6 and 1.0 x 10-5 mol L-1. Figure 6 shows the representative voltammograms and a linear standard
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addition curve for determination of 1.0 x 10-6 mol L-1 MP (sample 1). There was good
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precision and accuracy in the ability to recover MP in all drinking water samples. Table 3 shows the results obtained for MP determination at three levels of added concentration. Very
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satisfactory recovery values were found (varying between 102 to 110 %), suggesting that the method was not influenced by interfering substances in the analyzed samples (t-test at 95 %
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confidence).
4. Conclusions
TABLE 3
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FIGURE 6
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For the first time, activated biochar was incorporated in a carbon paste electrode and used for the development of an electrochemical sensor with excellent analytical performance. The proposed device showed better efficiency in spontaneous preconcentration of MP when compared to the other electrodes (carbon paste unmodified or modified with activated charcoal and untreated biochar). This is due to the high amount of acid functional groups present in the surface of biochar after chemical activation, as evidenced by characterization 13
ACCEPTED MANUSCRIPT analysis. The CPME with activated biochar showed good linearity and sensitivity comparable to other studies in the literature for the determination of MP. The device capability was confirmed by good accuracy of the developed method for the spontaneous preconcentration and determination of MP in drinking water samples. The results enable the proposal of
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developing a passive sampler for quantification of MP in aqueous samples.
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ACCEPTED MANUSCRIPT Acknowledgements We gratefully acknowledge financial support from Brazilian foundations: CAPES and CNPq. The authors would also like to thank the Academic Publishing Advisory Center (Centro de Assessoria de Publicação Acadêmica, CAPA - www.capa.ufpr.br) of the Federal University of
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Paraná for assistance with English language editing.
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[36] Y. Zeng, D. Yu, Y. Yu, T. Zhou, G. Shi, Differential pulse voltammetric determination
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pesticides on untreated and phosphoric acid treated biochar and charcoal from water, Journal
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adsorption capacity and mechanism, Bioresour. Technol. In Press, Accepted Manuscript, Available online 8 May 2017.
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ACCEPTED MANUSCRIPT [64] J. Mehta, P. Vinayak, S.K. Tuteja, V.A. Chhabra, N. Bhardwaj, A. Paul, K.-H. Kim, A. Deep, Graphene modified screen printed immunosensor for highly sensitive detection of parathion, Biosensors and Bioelectronics 83 (2016) 339-346. [65] H. Parham, N. Rahbar, Square wave voltammetric determination of methyl parathion using ZrO 2-nanoparticles modified carbon paste electrode, Journal of Hazardous Materials
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detection of methyl parathion based on a sensing platform constructed by the direct growth of
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carbon nanotubes on carbon paper, RSC Advances 6(63) (2016) 58771-58779.
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ACCEPTED MANUSCRIPT List of Figures Figure 1 – Differential pulse voltammograms for CPE unmodified, CPE modified activated carbon (CPE-AC) and CPE modified with biochar (CPME). In detail: variation of cathodic peak current for each electrode. Preconcentration conditions: acetate buffer solution pH 5.0
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containing 1.0 x 10-4 mol L-1 MP and preconcentration time of 5 minutes in the absence of applied potential. Measurements conditions: acetate buffer solution pH 5.0, pulse amplitude
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-90
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-1.0 -0.8 -0.6 -0.4 -0.2 -1 E / V (vs. Ag/AgCl 3.0 mol L KCl)
B
60
40
Ipc
D
-30
-60
80
M / A A
A
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I / A
0
CPE CPE-CA CPME CPME-AB
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of 100 mV s-1 and pulse time modulation of 100 ms.
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0
CPE
CPE-CA CPME CPME-AB
Electrodes
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Figure 2 – Possible adsorption mechanisms between biochar surface and methyl parathion.
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ACCEPTED MANUSCRIPT Figure 3: SEM images for the precursor and activated biochar samples in an increase of 300 times (A and C) and 2000 times (B and D), respectively. Instrumental parameters: FEI
C
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B
D D
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D
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A
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QUANTA 450 FEG instrument, with accelerating voltage of 10.0 kV.
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ACCEPTED MANUSCRIPT Figure 4 – Correlation curves between the peak current intensity and different preconcentration times (0 to 20 min) at range concentrations of 1.0 x 10-6 (■), 5.0 x 10-5 (▲) and 1.0 x 10-5 mol L-1 (●) MP. Preconcentration step performed in the absence of potential
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application.
-1
-5
-1
-6
-1
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10
1.0 x 10 mol L
D
1.0 x 10 mol L
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0 0
10
20
30
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Preconcentration time / min
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Ipc / A
-5
5.0 x 10 mol L
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ACCEPTED MANUSCRIPT Figure 5 – Analytical curves for different concentrations of MP using the modified electrodes with precursor (■) and activated biochar (●). Preconcentration conditions: acetate buffer solution pH 5.0 containing MP, preconcentration time of 5 minutes. Measurements conditions: acetate buffer solution pH 4.0, pulse amplitude of 100 mV s-1 and pulse time
A
60
pc
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I / A
-25
-0,8
-0,6
-0,4
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-1,0
D
-75
-100
B
NI U/ A SC R
80
IP
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modulation of 100 ms.
-0,2
-1
20
0
0
20
40
60
80 100
CMP / mol L
-1
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E / V (vs Ag/AgCl 3.0 mol L KCl)
40
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ACCEPTED MANUSCRIPT Figure 6 – Voltammograms and standard addition curve (in detail) obtained for determination of 1.0 x 10-6 mol L-1 MP in drinking water sample.
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Ipc / A
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-40
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20 15 10
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0
2
4
CMP / mol L
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-1
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-0,8 -0,6 -0,4 -0,2 0,0 -1 E / V (vs. Ag/AgCl 3.0 mol L KCl)
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Blank Sample Standard
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ACCEPTED MANUSCRIPT LIST OF TABLES
Table 1 – Semi-quantitative elemental composition of biochar samples by EDS analysis.
Biochar sample
a
C
N
O
Mg
Al
Si
Precursor
64.4
6.90
19.9
1.50
1.70
0.70
Activated
60.6
10.3
24.6
0.10
2.60
N A
not detected.
D E
S
K
Ca
0.79
0.50
1.40
2.30
a
0.20
a
a
C S U
1.60
T P
I R
Elemental composition (%)
P
M
T P E
C C
A
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ACCEPTED MANUSCRIPT Table 2 – Analytical performance for CPME with precursor and activated biochar and different electroanalytical methods applied for methyl parathion determination. LDR
Sensitivity
LOD
(µmol L-1)
(µA L µmol L-1)
(nmol L-1)
DPAdSV
0.10 – 70
CPME-PB
DPAdSV
0.10 – 50
I R
River water
MIP-NPAu
CV
0.34 – 34
Phosphate buffer
SPCE
CV
Water / Vegetables
GCE-Nafion/NPAu
CV
Drinking water
CPME-NPZrO2
Acetate buffer
CPME-NiTSPc/Nafion
Apples
GCE-NPTiO2
Sample
Electrode
Technique
CPME-AB
0.76
39
b
0.46
65
b
a
34
[24]
2.0 – 80
0.11
500
[61]
0.10 – 120
0.46
100
[62]
0.02 – 10
a
6.86
[65]
SWV
3.4 – 34
0.02
340
[66]
LSAdSV
5.0 – 100
0.82
1.0
[67]
DPAdSV
0.038 – 3.8
65
14.4
[68]
Drinking water
Kiwi fruit
C A
E C
CPE-CNTs
D E SWV
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T P
Ref.
C S U
M
N A
CPME: Carbon paste modified electrode; MIP: Molecular imprinted polymer; NP: nanoparticle; SPCE: Screen printed carbon electrode; GCE: Glassy carbon electrode; CPE: Carbon paper electrode; CNTs: Carbon nanotubes. a not reported b present work
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Table 3 – Added and recovered concentration values for spiked drinking water samples with different concentrations of MP. Add (mol L-1)
Founded (mol L-1)
Recovery (%)
1
1.00 x 10-6
1.02 x 10-6
102 ± 2.1
2
5.00 x 10-6
5.31 x 10-6
106 ± 1.2
3
1.00 x 10-5
1.10 x 10-5
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Sample
AC
CE
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D
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110 ± 3.3
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Graphical Abstract
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ACCEPTED MANUSCRIPT Highlights
Use of activated biochar as new electrode modifier for determination of
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methyl parathion in aqueous samples
Alternative electrochemical sensor for methyl parathion determination
A simple chemical route to improve the adsorption ability of biochar
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Morphological and structural characterization of surface proprieties of
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treated and untreated biochar
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towards methyl parathion
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Paulo Roberto de Oliveira, Cristiane Kalinke, Jeferson Luiz Gogola, Antonio Salvio Mangrich, Luiz Humberto Marcolino Junior, Márcio F. Bergamini PII: DOI: Reference:
S1572-6657(17)30448-4 doi: 10.1016/j.jelechem.2017.06.020 JEAC 3351
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
13 February 2017 15 May 2017 12 June 2017
Please cite this article as: Paulo Roberto de Oliveira, Cristiane Kalinke, Jeferson Luiz Gogola, Antonio Salvio Mangrich, Luiz Humberto Marcolino Junior, Márcio F. Bergamini , The use of activated biochar for development of a sensitive electrochemical sensor for determination of methyl parathion, Journal of Electroanalytical Chemistry (2017), doi: 10.1016/j.jelechem.2017.06.020
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ACCEPTED MANUSCRIPT The use of activated biochar for development of a sensitive electrochemical sensor for determination of methyl parathion
Paulo Roberto de Oliveiraa, Cristiane Kalinkea, Jeferson Luiz Gogolaa, Antonio Salvio
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Mangrichb,c, Luiz Humberto Marcolino Juniora, Márcio F. Bergaminia*
a- Laboratório de Sensores Eletroquímicos (LabSensE), Departamento de Química,
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Universidade Federal do Paraná (UFPR), CEP 81.531-980, Curitiba-PR, Brazil.
b- Laboratório de Química de Húmus e Fertilizantes, Departamento de Química, Universidade
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Federal do Paraná (UFPR), CEP 81.531-980, Curitiba-PR, Brazil.
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c- Instituto Nacional de Ciência e Tecnologia de Energia e Ambiente (INCT E&A/CNPq),
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Brazil.
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*email address: [email protected]
Telephone number: +55 41 3361-3177
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Fax number: +55 41 3361-3186
ACCEPTED MANUSCRIPT
Abstract Biochar is a carbonaceous material that exhibits rich surface functional groups, which makes its use as a modifier of electrodes a very attractive alternative, especially the preconcentration of species of interest, organic or inorganic. This capability can be enhanced
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by activation of the material that increases the amount of functional groups, hence the ability to preconcentrate more species. In this study we used a carbon paste electrode modified with
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biochar activated with nitric acid for spontaneous preconcentration of Methyl Parathion and
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for further quantitative determination in drinking water. The electrode showed good
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sensitivity and limits of detection of MP, 760 µA L mmol-1, 39.0 nmol L-1, respectively. Good accuracy of the method to determine MP in tap water samples was observed with recoveries
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values varying between 102 to 110 % for three different concentration levels.
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Keywords: Activated biochar, methyl parathion, electrochemical sensor, spontaneous
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preconcentration.
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1. Introduction Biochar is a material which is rich in carbon and is produced by pyrolysis of biomass at moderate temperature conditions (< 700 ºC) and low amount or absence of oxygen [1]. The characteristics of this material simulate those observed for the “Terra Preta de Índio da
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Amazônia”, which consist of an inert internal structure and a highly functionalized surface with the ability to interact with different compounds [2-4]. The addition of other species on
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biochar can promote greater surface functionalization, forming an activated material with
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unique characteristics [5]. In general, this property makes the use of biochar advantageous in
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removing organic and inorganic contaminants in soil and water matrices [6-8]. Chemical activation of biochar is a commonly used method [9, 10]. For this reason, chemical agents are
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used for modifying the carbonaceous material surface, such as nitric acid [11], sodium hydroxide [12], hydrogen peroxide [13], among others.
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Some studies have reported that the use of biochar as a modifier of electrodes can
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improve the capacity of preconcentration of metallic ions at low limits of determination [1417]. For example, Kalinke et al. [18] report the quantification of paraquat using a carbon paste
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electrode modified with biochar. However, the determination of organic compounds using electrodes modified with biochar still requires further exploration. Nevertheless, the improved
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detectability of the proposed device in comparison to unmodified electrodes has been attributed to the presence of biochar. This is due to the ability of biochar to interact with cations of paraquat molecule. The biochar surface has negative charges which combine with the positively charged paraquat cations by electrostatic interactions [19]. Thus, the proposed method presents sensitivity of 19.2 µA L µmol-1 and limit of detection of 7.5 nmol L-1. The paraquat determination in spiked samples presents recovery values of 95.8 % and 97.5 % for coconut and natural water. 1
ACCEPTED MANUSCRIPT The interaction between biochar and other pesticides, such as methyl parathion (MP), has constantly been reported in the literature [20, 21]. The sorption of this pesticide on biochar was related to combined mechanisms of π–π interaction between the aromatic molecule of MP and the surface of biochar [19, 22], as well as interactions between the MP nitrophenyl group and the functional groups of biochar by hydrogen bonds [23, 24]. Methyl
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Parathion (O,O-dimethyl O-4-nitrophenyl phosphorothioate) is an organophosphate
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insecticide [25]. MP is highly toxic by inhalation and ingestion, and moderately toxic by
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dermal absorption [26, 27]. The US-EPA classifies it as toxicity class I, the most toxic, and its use has been banned in the EU since 2005 [28]. In Brazil, MP is largely employed to combat
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pests in many crops, mainly vegetables, fruits and cereals [29]. This pesticide is usually determined by gas chromatography (GC) [30, 31], liquid chromatography (HPLC) [32, 33]
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and voltammetric analysis, mostly using differential pulse or square-wave voltammetry [3437].
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Since there are no reports of the analytical performance of electrodes modified with
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activated biochar, and MP exhibits a good adsorption in biochar, the present paper reports for the first time the use of voltammetric determination of MP using carbon paste modified
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electrode (CPME) with biochar activated with HNO3.
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2. Material and methods
2.1 Materials and Chemicals Powdered graphite and mineral oil were acquired from Merck and Fisher Scientific, respectively (carbon paste), the standard of methyl parathion from Sigma Aldrich. Analytical grade glacial acetic acid (Isofar), anhydrous sodium acetate (J. T. Baker), acetonitrile (Sigma Aldrich), hydrochloric acid (F. Maia) and sodium hydroxide (Cromato) were used as received. All solutions were prepared with deionized water obtained with a Millipore Milli-Q system. 2
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2.2 Preparation and characterization of biochar sample First, precursor biochar samples were obtained from castor oil cake biomass for pyrolysis process at temperature of 400 ºC, heating rate of 5 ºC min-1 and residence time of 60 minutes. The sample was subjected to surface treatment process by chemical activation in
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order to increase the amount of surface functional groups and improve the preconcentration
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capacity of the material. For the surface treatment, HNO3 solution 50 % (v/v) was added to 1 g of precursor biochar. The dispersion was placed in a reflux system under constant stirring in
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temperature at 60 ºC for 3 hours. After the chemical activation step, the dispersion was
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filtered, washed with distilled water, dried in an oven at 100 °C for 24 hours, and stored for
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later use as modifier agent of electrodes.
2.3 Electrode fabrication
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The carbon pastes employed in this work consisted of powdered graphite, mineral oil
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and the modifier (biochar). The proportion utilized was 25% (w/w) of mineral oil, 75–X% of graphite and X% (w/w) of biochar in the ratios of X = 5, 10, 15, 25, 30 and 50%. Pastes were
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prepared by simple mixing and homogenization of the components. The carbon paste electrode was made by compacting of carbon paste into a plastic cylinder with an internal
support.
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diameter of 3 mm and a copper wire (electrical connection) with similar thickness to plastic
2.4 Characterization Measurements The methyl parathion electrochemical characterization was performed by cyclic voltammetry employ of the carbon paste electrode modified with biochar in presence of methyl parathion (1.0 x 10-4 mol L-1). Cyclic voltammograms (n = 5) with scan rate of 50 mV 3
ACCEPTED MANUSCRIPT s-1 were obtained and the initial scan was taken from 0.4 V to -1.2 V. For the voltammetric measurements a 10.0 mL voltammetric cell with the conventional three-electrode configuration was employed, in which the working, auxiliary and reference electrodes were CPE unmodified, and CPME with activated carbon (AC) and biochar, Pt wire and Ag/AgCl in 3.0
mol
L-1
KCl
respectively.
The
results
were
performed
on
an
AutoLab
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potentiostat/galvanostat (Metrohn), with data acquisition and experimental control by the
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NOVA software version 2.0.
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2.5 Voltammetric analysis
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The analytical procedure for determination of the MP occurred in three steps: (1) Preconcentration of MP on the electrode surface (open circuit) by stirring for five minutes in
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0.1 mol L-1 acetate buffer solution (pH 5) content 1.0 x 10-4 mol L-1 of MP; (2) Voltammetric analysis of the electrode containing MP adsorbed in 0.1 mol L-1 acetate buffer solution (pH 4)
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by differential pulse cathodic stripping voltammetry (DPCSV), the range potential being
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subsequently held from 0.4 V to -1.2 V, pulse amplitude of 100 mV s-1, and pulse time modulation of 100 ms; and (3) Renewal of voltammetric surface by methyl parathion removal
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from polishing the surface.
Experimental parameters were investigated for each step involved in the
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preconcentration and determination of MP, such as preconcentration and measuring solutions, electrode construction conditions and instrumental parameters of DPV technique.
2.6 Validation of the analytical method The analytical validation of the proposed method was done for the carrying out of the analytical curve, presence of concomitant species and determination of MP in tap water spiked samples. For concomitant species studies, copper (II), iron (III) and zinc (II) ions, 4
ACCEPTED MANUSCRIPT ascorbic acid and glucose were used at three different concentrations: 1.0 x 10-6, 1.0 x 10-5 and 1.0 x 10-4 mol L-1. The species were separately added into preconcentration solution in presence of 1.0 x 10-5 mol L-1 MP. The effect of the interfering species was evaluated by comparing the response signal of MP. The determination of MP in the spiked sample was determined by the standard addition method with four successive additions. The voltammetric
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measurement was performed using the CPME and with previously optimized experimental
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conditions.
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3. Results and discussion
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3.1 Voltammetric evaluation of the CPME with biochar for Methyl Parathion preconcentration
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The evaluation of biochar as a modifier was performed through a study comparing the ability to preconcentrate methyl parathion for three different electrodes: unmodified carbon
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paste electrode (CPE), carbon paste electrode modified with activated carbon (CPE-AC) and
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carbon paste electrode modified with biochar (CPME). The three electrodes were submitted to the same process of preconcentration in pH 5.0 acetate buffer solution containing 1.0 x 10 -4
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mol L-1 MP in the absence of applied potential and constant stirring for 5 minutes.
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Voltammograms obtained with this evaluation are shown in Figure 1. FIGURE 1
Based on voltammograms in Figure 1, it is possible to affirm that the three evaluated electrodes exhibit the adsorbing capacity of the MP because of the observed peak current at 0.6 V in all electrodes studied. The ability to preconcentrate MP of the unmodified electrode may be associated with the existence of π-π interactions between the graphite and the surface of the aromatic ring present in the MP molecule [19, 20]. For the electrode modified with activated carbon, the strongest signal can be attributed to the presence of pores in the coal 5
ACCEPTED MANUSCRIPT surface (physical adsorption) and also by the existence of π-π interactions as mentioned above, since there is the presence of 50% (w/w) of graphite in the composition of this electrode. The results obtained using the electrodes modified with the biochar showed greater intensity of current compared to the unmodified electrode and the activated carbon modified
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electrode. The higher ability of the biochar towards preconcentration of methyl parathion on
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the electrode surface is related with functional groups present at biochar surface when
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compared to graphite and activated carbon. We believe that the mechanism of interaction between the MP and the surface of the electrode modified with biochar is similar to that
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proposed by Liu et al [38] concerning the adsorption of phenol by biochars that present great amounts groups that contain nitrogen or oxygen. According to the authors, the interaction
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between the organic molecule and the surface of biochar can mainly occur in three ways: the first (1) is the physical adsorption [39]; the second (2) is attributed to π-π interactions between
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the aromatic ring of the analyte and biochar aromatic structures (hydrophobic interactions)
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[39, 40], and the third and most important (dominant interaction mechanism) (3) is the chemical interaction of the organic molecules with the functional groups on the adsorbent
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material surface. In this case between nitrophenyl group of MP and carboxyl and hydroxyl groups of biochar (hydrogen bonds) [38, 41]. Thus the modified electrode with biochar can
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present different adsorption mechanisms with the MP which together can result in a combined effect improving the response signal of sensor [38, 39, 42] (Figure 2). FIGURE 2 However, it was observed that CPME with activated biochar showed better analytical performance in comparison to the precursor. This can be due to differences in physicochemical characteristics of biochar samples after chemical treatment, thus 6
ACCEPTED MANUSCRIPT necessitating morphological characterization by scanning electron microscopy (SEM), semiquantitative elemental composition by energy dispersive spectroscopy (EDS), and estimative of surface acid groups by the Boehm method.
3.1.1 Morphological characterization
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Scanning electron microscopy (SEM) analyses were performed so as to evaluate the
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morphological characteristics of biochar samples. The representative SEM images for samples
3.
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FIGURE 3
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of precursor biochar (untreated) and after chemical treatment (activated) are shown in Figure
From the SEM images, it is possible to observe some morphological variations
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between samples. Among these modifications is a small decrease of particle size with the employment of chemical treatment (Figure 3-A, C). Some works in the literature [43, 44]
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report that acid chemical treatment may cause a significant decrease of pore volume.
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Furthermore, some authors suggest that the use of long reflux times may also affect the structure of materials [45]. These morphological variations can be explained by the oxidation
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promoted by the HNO3, resulting in the formation of functional groups on the walls or edges of pore opening, as shown in Figure 3 (B, D) [46, 47]. Thus, it is possible to suggest that a
D).
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partial “obstruction” of the pores occurs after the biochar activation, as shown in Figure 3 (B,
3.1.2 Chemical composition The chemical mapping performed using an energy dispersive spectroscopy Thermo 200 coupled with SEM allows the elemental composition quantification of the samples exposed to the electron beam [48]. Surface chemical analysis by EDS was performed for 7
ACCEPTED MANUSCRIPT precursor and activated biochar samples, and from the results obtained it was possible to identify and quantify (semi-quantitatively) the elemental composition of the samples (Table 1). TABLE 1 From the EDS results, the existence of characteristic elements of biochar was observed
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in both samples [49, 50]. A material rich in carbon, with significant amounts of oxygen and
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nitrogen in its composition can be seen. Mineral compounds, such as magnesium, silicon,
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sulfur, potassium and calcium are also commonly found in the product of the pyrolysis and are derived from the feedstock. Thus, the elemental composition may be dependent on the
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biomass adopted [51].
For the activated sample, a poorer elemental composition was observed, possibly due
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to the fact that other elements may have been removed by way of the chemical treatment. From the composition of the samples it was possible to see variations consistent with other
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reports in the literature [46, 52]. Among these, one can emphasize the reduction of carbon in
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activated biochar compared to the precursor, which might have been caused by the formation of surface oxygen groups in the biochar and also for some mineralization of carbonaceous
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matrix [53]. The formation of these functional groups may occur in the aliphatic portion of the molecule by breaking the benzylic carbons of C–C bonds or by oxidation reactions involving
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methylene (–CH2) [54]. Parallel to this, nitrate ions in an acid medium are good oxidizing agents, and can promote the oxidation of the biochar surface, leading to formation of hydroxyl and carboxylic groups and increased oxygen levels. The increased nitrogen content may be related to the formation of nitro groups. The dissociation of HNO3 can generate the nitronium ions, which reacts with the aromatic rings of the biochar forming nitrated products (–NO2) superficially adsorbed [55]. In addition, this treatment can release ions from groups existing in 8
ACCEPTED MANUSCRIPT biochar, H+ ions can displace and/or solubilize cations that are present in the biochar surface. This allows functional groups of material to be available to interact with other compounds [56, 57].
3.1.3 Estimation of Functional groups
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Boehm titration [58, 59] was performed to estimate the total acid groups (carboxylic
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acid, lactonic and phenolic), in equivalent per gram of biochar (mmol g-1) present on the surface of the samples. The potentiometric titrations were performed with 0.1 mol L-1 NaOH
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as titrant and using a Metrohm pH meter 780. An increase of total acid groups from 4.98 ±
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0.29 to 7.87 ± 0.16 mmol g-1 was observed for precursor and activated biochar, respectively. The data show the existence of a greater amount of total acid groups on the surface of biochar
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after the chemical treatment performed. These results are in concordance with other reports in the literature [43]. This means that the chemical treatment with HNO3 was effective in the
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incorporation of oxygenated functional groups on the biochar.
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With the results from characterization analyses, differences among biochar samples before and after chemical treatment were detectable. FTIR spectrum (Figure S1) revealed the
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presence of different surface functional groups for activated biochar in comparison to the precursor biochar,, which is consistent with the increase of the oxygen and nitrogen
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composition. To verify the influence of porosity and surface area on the adsorptive capacity of the materials analysis by B.E.T. method were carried out. The activated biochar showed an increase of surface properties with a pore volume increase of 80 % in comparison to the precursor material (Figure S2). This means that the porosity of the activated biochar may have corroborated to the better performance of the MP preconcentration. As a result, the best response was obtained with voltammetric electrode modified with the treated biochar. This 9
ACCEPTED MANUSCRIPT demonstrates the use of this material as an electrodes modifier, in order to increase the device’s capacity of spontaneous preconcentration and the determination of species with high sensitivity and detectability.
3.2 Effect of the Experimental Parameters on the Voltammetric Response
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The ability of biochar to adsorb MP has already been well attested elsewhere in the
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literature. Thus, the study to assess the best composition of the modified carbon paste with biochar is extremely important to obtain the best voltammetric response. The evaluation was
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made using carbon pastes prepared with 0 to 50% (w/w) of biochar content under DPAdS
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voltammetry conditions in pH 5.0 acetate buffer solution containing 5.0 x 10-5 mol L-1 to MP time of 5 minutes at open circuit potential condition.
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There was a significant increase in the intensity of MP reduction current when the amount of biochar was escalated to 10% (w/w). The elevated amount of biochar leads to an
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increase in functional groups available for interaction with MP in the folder, and these are
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mainly carboxyl, hydroxyl, lactonic and phenolic groups [41, 60]. For quantities greater than 10% (w/w) the current intensity becomes practically constant, which defines this as the best
CE
composition of the carbon paste.
In order to obtain higher sensitivity and shorter analysis time, we evaluated the best
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contact time of CPME in the preconcentration solution with MP. In this study, different solutions of MP were used (1.0 x 10-6, 1.0 x 10-5 and 5.0 x 10-5 mol L-1), time of immersion 0.5 to 20 minutes in acetate buffer pH 5 solution (Figure 4). FIGURE 4 For the three concentrations evaluated, the same tendency of increase of current was observed, compared to the time of preconcentration. Just as the preconcentration time raises 10
ACCEPTED MANUSCRIPT the peak current intensity, it also increases until its stabilization. This tendency occurs because the greater the exposure time of CPME solution containing the MP, the more species are preconcentrated and thus the larger is the signal. The stabilization of the signal occurs in 5 minutes and may be explained by the equilibrium between the adsorption capacity of the material with the amount of MP in solution.
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The parameters of the differential pulse technique directly influence the voltammetric
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response for MP. The optimized parameters were as follows: biochar proportion of 10 %
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(m/m), preconcentration time of 5 min in pH 5 acetate buffer solution, support electrolyte of pH 4 acetate buffer solution, pulse amplitude and duration of 100 mV and 100 ms,
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respectively.
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3.3 Analytical performance
An analytical curve was made by employing differential pulse CPME with the
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previously optimized experimental conditions. To that end, voltammograms were obtained
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(n=3) with different concentrations of MP. Analytical curves were obtained for CPME with activated (●) and not activated (■) biochar samples (Figure 5B). Figure 5A shows
FIGURE 5
AC
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representative voltammograms for CPME with activated biochar.
It is observed that CPME-AB analytical performance was higher than obtained with the CPME-PB. This information is based on the highest sensitivity obtained for the treated biochar (0.76 µA L µmol-1) compared to the untreated (0.46 µA L µmol-1), and also from the LDR, being 0.1 to 70 µmol L-1 for the CPME-AB and 0.1 to 50 µmol L-1 for the CPME-PB. This better performance can be attributed to the greater amount of functional groups present 11
ACCEPTED MANUSCRIPT on the activated material. In other words, it is possible to establish a relation between increase of functional groups of the biochar surface and the improvement in the preconcentration capacity of methyl parathion at electrode surface. Chemical treatment with HNO3 can generate some significant changes on biochar surface as the increase of oxygen and nitrogen chemical groups which promote an improvement in the sensor response.
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Analytical performance for proposed electrodes (CPME-AB and CPME-PB) shown to
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be consistent with other works reported in the literature for the determination of methyl
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parathion, with good LDR, LOD and sensitivity (Table 2). Furthermore, most of the other methodologies reported to the MP determination in the measure solution (support electrolyte)
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[24, 61, 62]. On the other hand, in this work the preconcentration step is spontaneous, without electric potential application. This can minimize assay interference of concomitant species
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and can also enable the development of a passive sampler of methyl parathion. In addition, it should be emphasized that the proposed method presents a greater simplicity and lower cost
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in the construction of the electrodes compared to other works [63, 64]. TABLE 2
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3.4 Effect of concomitant species and applicability of the proposed sensor The results qualify the proposed method for the determination of MP in real samples,
AC
such as drinking water and fruit juices. It is for this reason that the study was conducted in the presence of concomitant species. Copper (II), iron (III) and zinc (II) ions were evaluated, as well as ascorbic acid and glucose. For this study, voltammetric measurements were performed in the absence and presence of these species in the preconcentration solution. The concentration of MP used in all the measures was set at 1.0 x 10-5 mol L-1, and for the assessed concomitant species three different concentrations were used: 1.0 x 10-6; 1.0 x 10-5 12
ACCEPTED MANUSCRIPT and 1.0 x 10-4 mol L-1. Significant interference was observed for the measures carried out in the presence of iron (III) ions and ascorbic acid in concentrations above 1.0 x 10-5 mol L-1; due to competition for adsorptive sites biochar among these species and MP. In this case, it would require a pretreatment of samples containing these species before the quantification MP.
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In this study, the determination in spiked samples of drinking water was executed
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without any prior treatment at three different concentrations of MP: 1.0 x 10 -6; 5.0 x 10-6 and 1.0 x 10-5 mol L-1. Figure 6 shows the representative voltammograms and a linear standard
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addition curve for determination of 1.0 x 10-6 mol L-1 MP (sample 1). There was good
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precision and accuracy in the ability to recover MP in all drinking water samples. Table 3 shows the results obtained for MP determination at three levels of added concentration. Very
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satisfactory recovery values were found (varying between 102 to 110 %), suggesting that the method was not influenced by interfering substances in the analyzed samples (t-test at 95 %
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confidence).
4. Conclusions
TABLE 3
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FIGURE 6
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For the first time, activated biochar was incorporated in a carbon paste electrode and used for the development of an electrochemical sensor with excellent analytical performance. The proposed device showed better efficiency in spontaneous preconcentration of MP when compared to the other electrodes (carbon paste unmodified or modified with activated charcoal and untreated biochar). This is due to the high amount of acid functional groups present in the surface of biochar after chemical activation, as evidenced by characterization 13
ACCEPTED MANUSCRIPT analysis. The CPME with activated biochar showed good linearity and sensitivity comparable to other studies in the literature for the determination of MP. The device capability was confirmed by good accuracy of the developed method for the spontaneous preconcentration and determination of MP in drinking water samples. The results enable the proposal of
AC
CE
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SC
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developing a passive sampler for quantification of MP in aqueous samples.
14
ACCEPTED MANUSCRIPT Acknowledgements We gratefully acknowledge financial support from Brazilian foundations: CAPES and CNPq. The authors would also like to thank the Academic Publishing Advisory Center (Centro de Assessoria de Publicação Acadêmica, CAPA - www.capa.ufpr.br) of the Federal University of
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NU
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Paraná for assistance with English language editing.
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pesticides on untreated and phosphoric acid treated biochar and charcoal from water, Journal
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adsorption capacity and mechanism, Bioresour. Technol. In Press, Accepted Manuscript, Available online 8 May 2017.
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ACCEPTED MANUSCRIPT List of Figures Figure 1 – Differential pulse voltammograms for CPE unmodified, CPE modified activated carbon (CPE-AC) and CPE modified with biochar (CPME). In detail: variation of cathodic peak current for each electrode. Preconcentration conditions: acetate buffer solution pH 5.0
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containing 1.0 x 10-4 mol L-1 MP and preconcentration time of 5 minutes in the absence of applied potential. Measurements conditions: acetate buffer solution pH 5.0, pulse amplitude
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-90
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-1.0 -0.8 -0.6 -0.4 -0.2 -1 E / V (vs. Ag/AgCl 3.0 mol L KCl)
B
60
40
Ipc
D
-30
-60
80
M / A A
A
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I / A
0
CPE CPE-CA CPME CPME-AB
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of 100 mV s-1 and pulse time modulation of 100 ms.
20
0
CPE
CPE-CA CPME CPME-AB
Electrodes
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D
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Figure 2 – Possible adsorption mechanisms between biochar surface and methyl parathion.
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ACCEPTED MANUSCRIPT Figure 3: SEM images for the precursor and activated biochar samples in an increase of 300 times (A and C) and 2000 times (B and D), respectively. Instrumental parameters: FEI
C
PT
B
D D
AC
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D
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A
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QUANTA 450 FEG instrument, with accelerating voltage of 10.0 kV.
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ACCEPTED MANUSCRIPT Figure 4 – Correlation curves between the peak current intensity and different preconcentration times (0 to 20 min) at range concentrations of 1.0 x 10-6 (■), 5.0 x 10-5 (▲) and 1.0 x 10-5 mol L-1 (●) MP. Preconcentration step performed in the absence of potential
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application.
-1
-5
-1
-6
-1
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30
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20
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10
1.0 x 10 mol L
D
1.0 x 10 mol L
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0 0
10
20
30
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Preconcentration time / min
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Ipc / A
-5
5.0 x 10 mol L
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ACCEPTED MANUSCRIPT Figure 5 – Analytical curves for different concentrations of MP using the modified electrodes with precursor (■) and activated biochar (●). Preconcentration conditions: acetate buffer solution pH 5.0 containing MP, preconcentration time of 5 minutes. Measurements conditions: acetate buffer solution pH 4.0, pulse amplitude of 100 mV s-1 and pulse time
A
60
pc
-50
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I / A
-25
-0,8
-0,6
-0,4
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-1,0
D
-75
-100
B
NI U/ A SC R
80
IP
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modulation of 100 ms.
-0,2
-1
20
0
0
20
40
60
80 100
CMP / mol L
-1
AC
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E / V (vs Ag/AgCl 3.0 mol L KCl)
40
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ACCEPTED MANUSCRIPT Figure 6 – Voltammograms and standard addition curve (in detail) obtained for determination of 1.0 x 10-6 mol L-1 MP in drinking water sample.
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Ipc / A
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-40
-4
20 15 10
-2
0
2
4
CMP / mol L
6
-1
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-0,8 -0,6 -0,4 -0,2 0,0 -1 E / V (vs. Ag/AgCl 3.0 mol L KCl)
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D
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25
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-20
AC
I / A
-10
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Blank Sample Standard
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ACCEPTED MANUSCRIPT LIST OF TABLES
Table 1 – Semi-quantitative elemental composition of biochar samples by EDS analysis.
Biochar sample
a
C
N
O
Mg
Al
Si
Precursor
64.4
6.90
19.9
1.50
1.70
0.70
Activated
60.6
10.3
24.6
0.10
2.60
N A
not detected.
D E
S
K
Ca
0.79
0.50
1.40
2.30
a
0.20
a
a
C S U
1.60
T P
I R
Elemental composition (%)
P
M
T P E
C C
A
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ACCEPTED MANUSCRIPT Table 2 – Analytical performance for CPME with precursor and activated biochar and different electroanalytical methods applied for methyl parathion determination. LDR
Sensitivity
LOD
(µmol L-1)
(µA L µmol L-1)
(nmol L-1)
DPAdSV
0.10 – 70
CPME-PB
DPAdSV
0.10 – 50
I R
River water
MIP-NPAu
CV
0.34 – 34
Phosphate buffer
SPCE
CV
Water / Vegetables
GCE-Nafion/NPAu
CV
Drinking water
CPME-NPZrO2
Acetate buffer
CPME-NiTSPc/Nafion
Apples
GCE-NPTiO2
Sample
Electrode
Technique
CPME-AB
0.76
39
b
0.46
65
b
a
34
[24]
2.0 – 80
0.11
500
[61]
0.10 – 120
0.46
100
[62]
0.02 – 10
a
6.86
[65]
SWV
3.4 – 34
0.02
340
[66]
LSAdSV
5.0 – 100
0.82
1.0
[67]
DPAdSV
0.038 – 3.8
65
14.4
[68]
Drinking water
Kiwi fruit
C A
E C
CPE-CNTs
D E SWV
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Ref.
C S U
M
N A
CPME: Carbon paste modified electrode; MIP: Molecular imprinted polymer; NP: nanoparticle; SPCE: Screen printed carbon electrode; GCE: Glassy carbon electrode; CPE: Carbon paper electrode; CNTs: Carbon nanotubes. a not reported b present work
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Table 3 – Added and recovered concentration values for spiked drinking water samples with different concentrations of MP. Add (mol L-1)
Founded (mol L-1)
Recovery (%)
1
1.00 x 10-6
1.02 x 10-6
102 ± 2.1
2
5.00 x 10-6
5.31 x 10-6
106 ± 1.2
3
1.00 x 10-5
1.10 x 10-5
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Sample
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D
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110 ± 3.3
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Graphical Abstract
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ACCEPTED MANUSCRIPT Highlights
Use of activated biochar as new electrode modifier for determination of
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methyl parathion in aqueous samples
Alternative electrochemical sensor for methyl parathion determination
A simple chemical route to improve the adsorption ability of biochar
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Morphological and structural characterization of surface proprieties of
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treated and untreated biochar
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towards methyl parathion
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