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Improved method for quantification of persistent organic pollutants and its remediation using modified clays materials
Microchemical Journal 150 (2019) 104081
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Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Improved method for quantification of persistent organic pollutants and its remediation using modified clays materials
T
Alejandro G. Haroa, Helvécio C. Menezesa, Tatiana A. Ribeiro-Santosa, ⁎ Ana E. Burgos Castellanosb, Zenilda L. Cardeala, a
Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Avenida Antônio Carlos, 6627, 31270901 Belo Horizonte, MG, Brazil Grupo de Investigación en Química de Coordinación y Bioinorgánica, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Sede-Bogotá, Av. Cra 30 NW. 45-03, Bogotá, Colombia
b
ARTICLE INFO
ABSTRACT
Keywords: POPs analysis and remediation DI-SPME GC/MS Bentonite and vermiculite
This study reports a direct immersion solid-phase microextraction method (DI-SPME) for the analysis of persistent organic pollutants (POPs), especially pesticide endosulfan, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) present at trace levels in water samples. The identification and quantification was performed by gas chromatography coupled to mass spectrometry (GC/MS). The optimized and validated methodology showed LOQs for endosulfan α and endosulfan β of 916.0 and 126.0 ng L−1 respectively, and for 2,3,7,8-TCDD of 10.0 ng L−1. Subsequently bentonite and vermiculite materials modified with cetyltrimethylammonium (CTA+) (B3CTA, B7CTA and B15CTA; V3CTA, V7CTA and V15CTA) were prepared, characterized and studied for the remediation of POPs in water.
1. Introduction The Stockholm Convention on Persistent Organic Pollutants (POPs) [1], is a global treaty aimed to protect human health and the environment from POPs. POPs are organic compounds that are resistant to photolytic, biological and chemical degradation, due to their persistence in the environment; besides, they have the potential to bioaccumulate in human [2]. POPs includes endosulfan, a registered pesticide for use in a wide variety of vegetables and fruits, cotton, soy, coffee and tea, as well as ornamental plants and trees. It has been banned in over 55 countries due to its high toxicity [3] for humans and aquatic organisms, as well as its persistence in the environment. Endosulfan consists of a mixture of two biologically active stereoisomers, the endosulfan α isomer and the endosulfan β isomer. Polychlorodibenzo-p-dioxins (PCDDs) also belong to the POPs group of the Stockholm Convention and are produced naturally in forest fires or volcanic activity and involuntarily by industrial, municipal and domestic incineration and combustion processes [4]. Dioxins are highly toxic compounds at very low concentrations [5,6]. In this context, POPs should be monitored, quantified and remedied to protect human and environmental health. Sample preparation for POPs determination is one of the most
⁎
important steps of the analysis process, due to the variety and complexity of the matrices, in addition to the low level of these compounds concentration. Pesticides are conventionally prepared by methodologies such as liquid-liquid extraction (LLE) and solid phase extraction (SPE) [7,8]. In the last years, alternatives to these procedures have been reported such as solid phase microextraction (SPME) [9–11], stir bar sorptive extraction (SBSE) [12], liquid phase microextraction (LPME) [13] and single drop microextraction (SDME) [14]. Dioxins are conventionally prepared by the Soxhlet method [15]. Some alternatives to this method are pressurized liquid extraction (PLE) [16], solid phase extraction (SPE) [17], and liquid-liquid microextraction (LLME) [18]. In the adsorption studies, bentonite and vermiculite have shown a greater number of applications than other mineral adsorbents [19,20]. Bentonite is a nonmetallic clay mineral based mainly on montmorillonite, which consists of silicate tetrahedra (SiO4), bound to a 2:1 alumina layer of octahedral crystal structure. Vermiculite is a hydrosilylated filosilicate mineral of 2:1 layer structure capable of storing interchangeable cations between layers. The cations present between the layers are easily replaced by other cations, so the clay materials have a good performance in the adsorption of cationic contaminants by cation exchange [21–23]. One of the most used cations to prepare these modified materials is cetyltrimethylammonium (CTA+), which is inserted between the layers
Corresponding author. E-mail address: [email protected] (Z.L. Cardeal).
https://doi.org/10.1016/j.microc.2019.104081 Received 7 December 2018; Received in revised form 7 July 2019; Accepted 8 July 2019 Available online 09 July 2019 0026-265X/ © 2019 Published by Elsevier B.V.
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Table 1 DI-SPME experimental variables 24 two-levels full factorial design for POPs extraction. Experimental variables
Low level
Centre level
High level
T (°C) tex (min) pH Salting-out (% mv−1)
50 20 4.0 1.0
60 25 5.0 3.0
70 30 6.0 5.0
and the outer surface [24]. So, promising results may be expected using modified bentonite and vermiculite for the adsorption of different types of organic pollutants. In recent years, some works have been reported concerning the remediation of pesticides and other contaminants using clays as extraction material in different matrices, such as white wine [25], ground water [26] and soils [27]. The aim of this study was the development of a DI-SPME method for the simultaneous determination of endosulfan and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in water samples for a remediation study using bentonite and vermiculite modified with CTA+. These efficient clay materials were prepared and characterized. Besides, no previous studies related to simultaneously SPME extraction of these compounds have been reported. Identification and quantification was performed by gas chromatography coupled to mass spectrometry (GC/ MS).
Fig. 2. Pareto plot for 24 two-level full factorial experiment designs of the SPME method optimization.
was used at 1.3 mL min−1. The following oven temperature program was used: temperature started at 50 °C, held for 1 min, it was heated to 150 °C at a rate of 25 °C min−1, then to 200 °C at 10 °C min−1; finally, the oven was heated to 270 °C at a rate of 35 °C min−1 and held for 5 min. The temperature of the injector was maintained at 250 °C and the temperature of the ion source was 280 °C. The injection was performed in splitless mode and acquisition of data in Scan/SIM mode. A temperature controlled heating block was used for the study. Polydimethylsiloxane SPME fiber (PDMS 100 μm) was obtained from Supelco®(St. Louis, MO, USA). The characterization of clays was performed by elemental analysis, infrared spectroscopy (IR), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The elemental analysis to determine carbon, hydrogen and nitrogen contents in the materials were performed on a Perkin Elmer CHN-PE-2400 apparatus from Perkin Elmer (Massachusetts, USA). X-Ray diffraction measurements were performed on a XRD-7000XRay Diffractometer from Shimadzu (Kyoto, Japan) with Cu Kα tube
2. Materials and methods 2.1. Instrumentation The analyzes were carried out with an Agilent gas chromatograph system coupled to a mass spectrometer with a quadrupole mass analyzer (GC/MS-7890A/5975C), equipped with a DB-5MS column (30 m × 0.25 mm ID × 0.25 μm) and a split/splitless injector manufactured by Agilent J&W made in Palo Alto, CA, USA. Ultrapure helium
Fig. 1. Procedure for the evaluation of POPs remediation in water with modified clays. 2
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Fig. 3. Response surfaces for the variables extraction time and extraction temperature using DISPME GC/MS.
(Munich, Germany). A stock solution of endosulfan at 1000.0 mg L−1 in methanol was prepared from the solid standard. A standard solution of 2,3,7,8-tetrachlorodibenzo-p-dioxin in toluene at 10.0-μg mL−1 was purchased from Sigma-Aldrich® (Bellefonte, PA, USA). Working solutions were prepared at different concentrations for each class of analyte. All extractions were carried out in 20 mL vials for SPME containing magnetic stirring.
Table 2 Analytical figures of merit for the determination of endosulfan and 2,3,7,8TCDD in aqueous samples by DI-SPME GC/MS method. Analyte Endosulfan α Endosulfan β 2,3,7,8-TCDD a
Working range (μg L−1) 0.92–25.00 0.13–25.00 0.01–0.50
R2 0.9995 0.9923 0.9187
LOD (μg L−1)
LOQ (μg L−1)
0.533 0.088 0.006
0.916 0.126 0.010
Cv (%)a 6.0 5.9 7.6
2.3. Method optimization and validation
RSD measured at the midpoint of the working range.
To optimize the SPME method, all experiments were performed with standard samples solution containing 100.00 μg L−1 of endosulfan α and endosulfan β and 1.00 μg L−1 of 2,3,7,8-tetrachlorodibenzo-pdioxin using direct immersion of the SPME fiber. The first step consisted in a univariate optimization of PDMS fiber desorption time. Desorption times of 5, 7, 10, 12, 15 and 17 min were evaluated. Then, a univariate optimization of the solvent was performed. The following solvents were evaluated: methanol, ethyl acetate, acetone, isopropanol and a mixture (1:1) of methanol-ethyl acetate. In a second step a multivariate optimization of other extraction parameters in DI-SPME mode was developed. A study was conducted using a 24 two-level full factorial design (FFD) to investigate the variables: extraction time (tex), extraction temperature (T), pH and saltingout effect by percentage of NaCl. The tests were performed in duplicate and the central point in triplicate, totaling 35 experiments. The experimental values of the variables studied are presented in Table 1. The minimum and maximum levels of the parameters were selected based on the properties of the analyzed POPs and following the levels presented in other research studies [8,10,11]. The temperature was limited by 70 °C because extraction temperatures above 70 °C cause losses of low molecular weights POPs, such as some pesticides. On the other hand, temperatures below 30 °C make it difficult to extract the POPs of higher molecular weights, like chlorinated aromatic compounds. The volume of modifier was set at 100.0 μL. The minimum level of pH was set at 4.0 and the maximum level of NaCl at 5.0% m v−1 to avoid damaging the fiber used in the extraction. The Doehlert design was applied using the most significant variables with the objective of reaching the optimum region of the investigated area. The model fits were validated using the analysis of variance (ANOVA). The p-values smaller than 0.05 were considered significant. All the statistical analyzes were performed using the statistical package Statistica 8.0 for Windows Stat soft Inc. (Tulsa, OK, USA). The method validation was developed according to Eurachem Guide “The Fitness for Purpose of Analytical Methods” of 2014 [28], evaluating
Table 3 Elemental analysis of bentonite pure and intercalated with different levels of CTA+Br− plus vermiculite pure and intercalated with different levels of CTA+Br−. Material
C (%)
H (%)
N (%)
BE B3CTA B7CTA B15CTA VE V3CTA V7CTA V15CTA
0.36 9.74 19.19 27.17 0.14 12.00 14.00 15.00
1.33 2.08 3.53 4.99 1.10 2.73 3.13 3.22
0.04 0.61 1.19 1.67 0.10 1.02 1.02 1.00
(1.5406 Å), 30 kV, 30 mA, in a range of 2θ from 4 to 70°, speed of 4° min−1, with space of 0.05 and constant time of 0.75 s per increment, equipped with polycapillary optics with parallel focus and graphite monochromator. In order to identify the functional groups present in the materials, Fourier transform infrared (FTIR) absorption spectroscopy was performed on a PerkinElmer (Massachusetts, USA) FTIR GX, in the spectral range of 400 to 4000 cm−1, resolution of 4 cm−1 and 64 scans. Samples were prepared as pellets with KBr. The lamellar structures of the materials were verified by transmission electron microscopy (TEM) measurements. Sample preparation was performed by dispersing the samples in acetone and subsequently dripping onto a carbon screen, Holey carbon-200. Transmission electron microscopy images were obtained from a G2-20-Super Twin Tecnai FEI (Hillsboro, USA) equipment. 2.2. Reagents and solutions Endosulfan α and endosulfan β were obtained from Sigma-Aldrich 3
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A.G. Haro, et al.
Fig. 4. Transmission electronic microscopic images of pure and intercalated clays materials (BE, VE, B3CTA+ and V15CTA)+.
the following performance parameters: selectivity, limit of detection, limit of quantification, working range, linearity and precision.
vermiculites (V3CTA, V7CTA, V15CTA) were studied by varying the amount of the adsorbent material per volume of aqueous solution (0.25 to 0.10 mg mL−1), fixing the contact time of the material under stirring with the aqueous solution in 10 min. Two tests were performed for each material, 12 experiments in total. Fig. 1 shows the schematic method.
2.4. Intercalation of clays The intercalation with CTA+Br− in the BE (bentonite) and VE (vermiculite) materials was performed using three CTA+Br−/clay (m m−1) ratios: 0.75, 0.35 and 0.15, following the procedure: to a suspension of CTA+Br− (100 mL of deionized water) were added 2.0 g of bentonite or vermiculite. The mixtures were left under magnetic stirring for 24 h at room temperature. The materials were then washed with deionized water to remove excess CTA+Br− and the solids were oven dried at 60 °C for 12 h [29–31]. The CTA+Br−/clay ratios defined above are theoretical. After the characterization of the materials by thermal analysis and elemental analysis, the percentage of CTA+Br− real, by mass, was obtained in the samples. The prepared materials were named using the initial letter BE or VE clay, followed by the actual surfactant content, as for example, the bentonite intercalated with 15% CTA+Br− was named B15CTA [29–31].
3. Results and discussion 3.1. Analytical method optimization The results of univariate optimizations showed that 12 min was necessary to achieve a complete desorption. Among the different matrix modifying solvents tested, the mixture MeOH-AcOEt (200 μL in 1:1 ratio) showed optimized and reproducible responses for both analytes. The parameter used to evaluate the multivariate optimization was the geometric mean of the analytes peaks areas. Analysis of the Pareto plot (Fig. 2) allowed us to evaluate the importance of each variable at a 95% confidence level. The significant variables were time and temperature of extraction. The level of significance for each regression calculated by ANOVA showed that the model was well fitted. The optimum values for each independent variable were calculated using the adjusted models to the maximum point of the function, therefore, these values should be used in the experimental procedure for the analytes extraction. The response surfaces obtained from the regression models of Doehlert design are presented in Fig. 3. The most influential factors on the response were the extraction temperature and extraction time. The
2.5. Remediation study The remediation capacity of modified bentonite and vermiculite was evaluated using standard samples solution containing 15.00 μg L−1 of endosulfan α and endosulfan β and 0.25 μg L−1 of 2,3,7,8-tetrachlorodibenzo-p-dioxin. The bentonites (B3CTA, B7CTA, B15CTA) and 4
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Fig. 5. CTA+Br− intercalation processes in bentonite and vermiculite clays for POP removal.
Fig. 6. Percent recoveries of endosulfan α, endosulfan β and 2,3,7,8-TCDD extracted from water by different bentonite/solution ratios (0.25 to 1.00 mg mL−1).
best extraction temperature was the maximum evaluated in the range of 65 to 70 °C, and the best extraction time was achieved at 30 min. It was observed that pH of the solution and the salting-out effect are not statistically significant. A slight improvement in the extraction was observed in pH values close to 6 and low concentration of NaCl. Thus, to achieve the optimum conditions for extraction of all POPs studied within the experimental domain, it was necessary to establish the temperature and extraction time at the maximum levels, 65 °C and 30 min, respectively, with no presence of NaCl and pH set at 6.0.
proposed DI-SPME method, using the optimized conditions. The results can be seen in Table 2. The analytical curve was obtained by potable water samples spiked by analytes and analyzed in triplicate for at least 6 points of uniformly spaced concentrations. A linear relationship was observed between the peak areas and analytes concentration over the full range of the calibration curves, with determination coefficients above 0.91 (p value < 0.05). The limits of detection (LOD) and quantification (LOQ) were obtained following the Eurachem guide. The method presented a good precision, low relative standard deviation (% RSD) values for all analytes were observed in a study of intra-repeatability.
3.2. Analytical method validation The principal analytical figures of merit were studied to validate the 5
Microchemical Journal 150 (2019) 104081
A.G. Haro, et al.
Fig. 7. Percent recoveries of endosulfan α, endosulfan β and 2,3,7,8-TCDD extracted from water by different vermiculite/solution ratios (0.25 to 1.00 mg mL−1).
3.3. Clays characterization
in the synthesis increases the interlamellar space from 1.37 nm (pure bentonite) to 1.58, 2.09 and 2.18 nm for B3CTA, B7CTA and B15CTA, respectively. [30] When CTA+Br− is added to the pure vermiculite (d001 1.14 nm), the diffraction peaks of this phase shift again to values 2θ smaller indicating that there was an interlamellar expansion of 1.24, 1.23 and 1.22 nm for V3CTA, V7CTA and V15CTA, respectively. These results also suggest low ionic exchange capacity of vermiculite, compared to bentonite, since in all three materials the amount of intercalated surfactant was similar, as well as values of interplanar distance [31].
The pure clays and the materials obtained after the intercalation [30] were characterized by different techniques. 3.3.1. Elemental analysis The elemental analysis was carried out with the purpose of estimating the CTA+Br− intercalated content in the clays, as well as to study the mass losses involved. Table 3 shows the carbon, hydrogen and nitrogen contents for the pure clays and the intercalated samples with different levels of CTA+Br−. Bentonite and vermiculite present high cation exchange capacity due to the isomorphic substitution of the major cations by lower valence cations, such as Al3+ instead of Si4+ and Mg2+ instead of Al3+. The magnitude and the location of these substitutions in the tetrahedral or octahedral sheets, produce clays with different properties. In the bentonite, these substitutions occur mainly in the octahedral sheets, that is, between the layers where there are cations that balance the excess of negative charge, consequently, the interactions between the cations and the two tetrahedral sheets are weak, leading to a great interlamellate expansion. On the other side, most substitutions occur in tetrahedral sheets for vermiculite, limiting the potential of vermiculite to expand its interlamellar space. [31]
3.3.4. Transmission electron microscopy The transmission electron microscope images of bentonite and vermiculite materials shows that after the intercalation there were not significant changes in the morphologies of the materials when compared to their pure analogy. However, the trend of cluster formation with the presence of surfactant is apparent. Already the transmission electronic microscopic images shows (Fig. 4) the lamellae present in the clays, before and after the intercalation with CTA+Br−. Transmission electron microscope measurements for bentonite (BE), vermiculite (VE), CTA+Br− pure, and for intercalated clays with different CTA+Br− contents were used to evaluate the intercalation of CTA+Br− in clays [30,31]. Fig. 5 shows the CTA+Br− intercalation processes in bentonite and vermiculite clays for POP removal.
3.3.2. Infrared spectroscopy An analysis of vibrational spectroscopy of the infrared region was carried out to identify the functional groups presents in the clays materials, pure clays and intercalated with CTA+Br−. From the CTA+Br− spectra and from the clays intercalated with it, bands in the regions of 2850–2920 cm−1 and 1600 cm−1 are observed due to the –CH and –NH vibrations, respectively. The bands at 1490 and 1475 cm−1 can be attributed to the CH3 [–N+(CH3)3] groups of the CTA+Br−, corroborating for the intercalation of surfactant molecules between the layers of the clays. For the pure and intercalated clays with CTA+Br−, there are bands in the 3500 cm−1 region due to the presence of Si–OH; and vibrations at 1160–1080 cm−1 characteristic of Si–O–Al and Si–O–Si. The bands at 650 e 450 cm−1 are assigned to the SieO group [30,31].
3.4. Remediation study Figs. 6 and 7 show the results of the evaluation of POPs remediation in aqueous samples by modified bentonite and vermiculite. The percentage of extraction of each analyte was calculated according to Eq. (1).
E(%) =
TA
BA BA
× 100
(1)
where E (%) = Extraction percentage; BA = Blank area; TA = Target analyte area of test with adsorption material. Figs. 6 and 7 present excellent results for remediation of 2,3,7,8TCDD. The dioxin was completely removed from the prepared solutions at tested ratios of material/solution. Besides, for endosulfan the extraction percentages oscillate in the range of 85.0% to 100%, observing that increasing the % CTA+ increase the extraction level. It was also
3.3.3. X-ray diffraction The composite diffractograms show that adding CTA+Br− to the pure clay, provides the mineral phase peaks change to 2θ values smaller, indicating an interlamellar expansion of the structure due to the presence of CTA+Br−. The increase of the CTA+Br− amount used 6
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observed that in general the clay materials with acronyms 7CTA and 15CTA have similar percentages of extraction, which can indicate that the materials structure was not significantly modified with higher amount of cation.
[8] R. Bonansea, M. Amé, D. Wunderlin, Determination of priority pesticides in water samples combining SPE and SPME coupled to GC–MS. A case study: Suquía River basin (Argentina), Chemosphere 90 (2013) 1860–1869. [9] J. Robles, et al., Comparative evaluation of liquid–liquid extraction, solid-phase extraction and solid-phase microextraction for the gas chromatography–mass spectrometry determination of multiclass priority organic contaminants in wastewater, Talanta 117 (2013) 382–391. [10] R. Boussahel, et al., Determination of chlorinated pesticides in water by SPME/GC, Water Res. 36 (2002) 1909–1911. [11] J. Merib, et al., Simultaneous determination of trihalomethanes and organochlorine pesticides in water samples by direct immersion-headspace-solid phase microextraction, J. Chromatogr. A 1321 (2013) 30–37. [12] N. Ochiai, K. Sasamoto, H. Kanda, E. Pfannkoch, Sequential stir bar sorptive extraction for uniform enrichment of trace amounts of organic pollutants in water samples, J. Chromatogr. A 1200 (2008) 72–79. [13] L. Hou, G. Shen, H. Lee, Automated hollow fiber-protected dynamic liquid-phase microextraction of pesticides for gas chromatography–mass spectrometric analysis, J. Chromatogr. A 985 (2003) 107–116. [14] A. De Souza, G. Olimpo, J. Bittencourt, A SDME/GC–MS methodology for determination of organophosphate and pyrethroid pesticides in water, Microchem. J. 99 (2011) 303–308. [15] US EPA; Method 1613; United States Environmental Protection Agency, https:// well-labs.com/docs/epa_method_1613b_1994.pdf, (1994) , Accessed date: 26 April 2019. [16] M. Kishida, T. Maekawa, B. Hiroshi, Effect of extraction temperature on pressurized liquid extraction of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and dioxin-like polychlorinated biphenyls from a sediment sample using polar and non-polar solvent, Anal. Chim. Acta 659 (2010) 186–193. [17] J. Choi, J. Lee, B. Moon, K. Baek, Semi-automated disk-type solid-phase extraction method for polychlorinated dibenzo-p-dioxins and dibenzofurans in aqueous samples and its application to natural water, J. Chromatogr. A 1157 (2007) 17–22. [18] S. Dasgupta, et al., Extraction of pesticides, dioxin-like PCBs and PAHs in water based commodities using liquid–liquid microextraction and analysis by gas chromatography–mass spectrometry, J. Chromatogr. A 1218 (2011) 6780–6791. [19] S. Tahir, N. Rauf, Removal of a cationic dye from aqueous solutions by adsorption onto bentonite clay, Chemosphere 63 (2006) 1842–1848. [20] S. Hamid, M. Shahadat, S. Ismamil, Development of cost effective bentonite adsorbent coating for the removal of organic pollutant, Appl. Clay Sci. 149 (2017) 79–86. [21] Z. Huang, et al., Modified bentonite adsorption of organic pollutants of dye wastewater, Mater. Chem. Phys. 202 (2017) 266–276. [22] S. İşçi, Intercalation of vermiculite in presence of surfactants, Appl. Clay Sci. 146 (2017) 7–13. [23] O. Duman, S. Tunç, T. Polat, Determination of adsorptive properties of expanded vermiculite for the removal of C. I. Basic Red 9 from aqueous solution: kinetic, isotherm and thermodynamic studies, Appl. Clay Sci. 109–110 (2015) 22–32. [24] D. Chen, et al., Characterization of anion–cationic surfactants modified montmorillonite and its application for the removal of methyl orange, Chem. Eng. J. 171 (2011) 1150–1158. [25] D. Doulia, et al., Effect of clarification process on the removal of pesticide residues in white wine, Food Control 72 (2017) 134–144. [26] J. Li, et al., Adsorption of herbicides 2,4-D and acetochlor on inorganic–organic bentonites, Appl. Clay Sci. 46 (2009) 314–318. [27] Rodríguez-Cruz, et al., Modification of clay barriers with a cationic surfactant to improve the retention of pesticides in soils, J. Hazard. Mater. B139 (2007) 363–372. [28] Eurachem Guide: “The Fitness for Purpose of Analytical Methods”, https://www. eurachem.org/index.php/publications/guides/mv, (2014) , Accessed date: 26 April 2019. [29] B. Zohra, et al., Adsorption of direct red 2 on bentonite modified by cetyltrimethylammonium bromide, Chem. Eng. J. 136 (2) (2008) 295–305. [30] C. Burgos, T. Ribeiro-Santos, R. Lago, Adsorption of the harmful hormone ethinyl estradiol inside hydrophobic cavities of CTA+ intercalated montmorillonite, Water Sci. Technol. 74 (2016) 663–671. [31] C. Burgos, T. Ribeiro-Santos, R. Lago, Porous expanded vermiculite containing intercalated cetyltrimethylammonium: a versatile sorbent for the hormone ethinylestradiol from aqueous medium, Int. J. Environ. Sci. Technol. 15 (2018) 1–18.
4. Conclusions The method proposed in this study proved to be a good alternative to conventional sample preparation protocol described in the literature for the extraction of persistent organic pollutants from water. The univariate and multivariate optimizations allowed the optimization of effective conditions for the simultaneous extraction of endosulfan α, endosulfan β and 2,3,7,8-TCDD in DI-SPME mode. In addition, good results were obtained for the analytical figures of merit during validation. It has been shown that different classes of compounds, which are normally extracted under different conditions, may be extracted in a single assay by the developed method. On the other hand, modified clays were tested to evaluate their efficiency as a remediation material for POPs in water, and all materials tested showed excellent results, reaching for endosulfan α, endosulfan β and 2,3,7,8-TCDD extraction percentages > 85.0% in the studied conditions. Thus, these modified clays establish a greater advantage for the remediation of aqueous samples according to low cost and physicochemical properties that allow a greater adsorption of analytes. Acknowledgement The authors thanks the financial support of the Brazilian Institutions: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Ministério da Saúde. References [1] Stockholm Convention on Persistent Organic Pollutants (POPs), Text and Annexes; The Secretariat of the Stockholm Convention, 2, 3 (2010), pp. 33–56. [2] L. Ritter, K. Solomon, J. Forget, Persistent organic pollutants: an assessment report on DDT, aldrin, dieldrin, endrin, chlordane, heptachlor, hexachlorobenzene, mirex, toxaphene, PCBs, dioxins and furans, Report for the International Programme on Chemical Safety (IPCS) Within the Framework of the Inter-Organization Programme for the Sound Management of Chemicals (IOMC), 2005, pp. 7–18. [3] US EPA, Reregistration Eligibility Decision for Endosulfan; EPA-738-R-02-013, (2002), pp. 4–8. [4] ATSDR, Toxicological Profile for Chlorinated Dibenzo-p-dioxins, vols. 1-8, Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA, 1998, pp. 19–56. [5] J. Saurat, O. Sorg, et al., The Cutaneous lesions of dioxin exposure: lessons from the poisoning of Victor Yushchenko, Toxicol. Sci. 125 (2012) 310–317. [6] U.S. EPA, 2,3,7,8-Tetrachlorodibenzo-p-dioxin (2,3,7,8,-TCDD) Summary, Office of Research and Development, Cincinnati, OH, https://www.epa.gov/sites/ production/files/2016-09/documents/2-3-7-8-tetrachlorodibenzo-p-dioxin.pdf, (2000) , Accessed date: 26 April 2019. [7] M. Shamsipur, N. Yazdanfar, M. Ghambarian, Combination of solid-phase extraction with dispersive liquid–liquid microextraction followed by GC–MS for determination of pesticide residues from water, milk, honey and fruit juice, Food Chem. 204 (2016) 289–297.
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Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Improved method for quantification of persistent organic pollutants and its remediation using modified clays materials
T
Alejandro G. Haroa, Helvécio C. Menezesa, Tatiana A. Ribeiro-Santosa, ⁎ Ana E. Burgos Castellanosb, Zenilda L. Cardeala, a
Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Avenida Antônio Carlos, 6627, 31270901 Belo Horizonte, MG, Brazil Grupo de Investigación en Química de Coordinación y Bioinorgánica, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Sede-Bogotá, Av. Cra 30 NW. 45-03, Bogotá, Colombia
b
ARTICLE INFO
ABSTRACT
Keywords: POPs analysis and remediation DI-SPME GC/MS Bentonite and vermiculite
This study reports a direct immersion solid-phase microextraction method (DI-SPME) for the analysis of persistent organic pollutants (POPs), especially pesticide endosulfan, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) present at trace levels in water samples. The identification and quantification was performed by gas chromatography coupled to mass spectrometry (GC/MS). The optimized and validated methodology showed LOQs for endosulfan α and endosulfan β of 916.0 and 126.0 ng L−1 respectively, and for 2,3,7,8-TCDD of 10.0 ng L−1. Subsequently bentonite and vermiculite materials modified with cetyltrimethylammonium (CTA+) (B3CTA, B7CTA and B15CTA; V3CTA, V7CTA and V15CTA) were prepared, characterized and studied for the remediation of POPs in water.
1. Introduction The Stockholm Convention on Persistent Organic Pollutants (POPs) [1], is a global treaty aimed to protect human health and the environment from POPs. POPs are organic compounds that are resistant to photolytic, biological and chemical degradation, due to their persistence in the environment; besides, they have the potential to bioaccumulate in human [2]. POPs includes endosulfan, a registered pesticide for use in a wide variety of vegetables and fruits, cotton, soy, coffee and tea, as well as ornamental plants and trees. It has been banned in over 55 countries due to its high toxicity [3] for humans and aquatic organisms, as well as its persistence in the environment. Endosulfan consists of a mixture of two biologically active stereoisomers, the endosulfan α isomer and the endosulfan β isomer. Polychlorodibenzo-p-dioxins (PCDDs) also belong to the POPs group of the Stockholm Convention and are produced naturally in forest fires or volcanic activity and involuntarily by industrial, municipal and domestic incineration and combustion processes [4]. Dioxins are highly toxic compounds at very low concentrations [5,6]. In this context, POPs should be monitored, quantified and remedied to protect human and environmental health. Sample preparation for POPs determination is one of the most
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important steps of the analysis process, due to the variety and complexity of the matrices, in addition to the low level of these compounds concentration. Pesticides are conventionally prepared by methodologies such as liquid-liquid extraction (LLE) and solid phase extraction (SPE) [7,8]. In the last years, alternatives to these procedures have been reported such as solid phase microextraction (SPME) [9–11], stir bar sorptive extraction (SBSE) [12], liquid phase microextraction (LPME) [13] and single drop microextraction (SDME) [14]. Dioxins are conventionally prepared by the Soxhlet method [15]. Some alternatives to this method are pressurized liquid extraction (PLE) [16], solid phase extraction (SPE) [17], and liquid-liquid microextraction (LLME) [18]. In the adsorption studies, bentonite and vermiculite have shown a greater number of applications than other mineral adsorbents [19,20]. Bentonite is a nonmetallic clay mineral based mainly on montmorillonite, which consists of silicate tetrahedra (SiO4), bound to a 2:1 alumina layer of octahedral crystal structure. Vermiculite is a hydrosilylated filosilicate mineral of 2:1 layer structure capable of storing interchangeable cations between layers. The cations present between the layers are easily replaced by other cations, so the clay materials have a good performance in the adsorption of cationic contaminants by cation exchange [21–23]. One of the most used cations to prepare these modified materials is cetyltrimethylammonium (CTA+), which is inserted between the layers
Corresponding author. E-mail address: [email protected] (Z.L. Cardeal).
https://doi.org/10.1016/j.microc.2019.104081 Received 7 December 2018; Received in revised form 7 July 2019; Accepted 8 July 2019 Available online 09 July 2019 0026-265X/ © 2019 Published by Elsevier B.V.
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Table 1 DI-SPME experimental variables 24 two-levels full factorial design for POPs extraction. Experimental variables
Low level
Centre level
High level
T (°C) tex (min) pH Salting-out (% mv−1)
50 20 4.0 1.0
60 25 5.0 3.0
70 30 6.0 5.0
and the outer surface [24]. So, promising results may be expected using modified bentonite and vermiculite for the adsorption of different types of organic pollutants. In recent years, some works have been reported concerning the remediation of pesticides and other contaminants using clays as extraction material in different matrices, such as white wine [25], ground water [26] and soils [27]. The aim of this study was the development of a DI-SPME method for the simultaneous determination of endosulfan and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in water samples for a remediation study using bentonite and vermiculite modified with CTA+. These efficient clay materials were prepared and characterized. Besides, no previous studies related to simultaneously SPME extraction of these compounds have been reported. Identification and quantification was performed by gas chromatography coupled to mass spectrometry (GC/ MS).
Fig. 2. Pareto plot for 24 two-level full factorial experiment designs of the SPME method optimization.
was used at 1.3 mL min−1. The following oven temperature program was used: temperature started at 50 °C, held for 1 min, it was heated to 150 °C at a rate of 25 °C min−1, then to 200 °C at 10 °C min−1; finally, the oven was heated to 270 °C at a rate of 35 °C min−1 and held for 5 min. The temperature of the injector was maintained at 250 °C and the temperature of the ion source was 280 °C. The injection was performed in splitless mode and acquisition of data in Scan/SIM mode. A temperature controlled heating block was used for the study. Polydimethylsiloxane SPME fiber (PDMS 100 μm) was obtained from Supelco®(St. Louis, MO, USA). The characterization of clays was performed by elemental analysis, infrared spectroscopy (IR), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The elemental analysis to determine carbon, hydrogen and nitrogen contents in the materials were performed on a Perkin Elmer CHN-PE-2400 apparatus from Perkin Elmer (Massachusetts, USA). X-Ray diffraction measurements were performed on a XRD-7000XRay Diffractometer from Shimadzu (Kyoto, Japan) with Cu Kα tube
2. Materials and methods 2.1. Instrumentation The analyzes were carried out with an Agilent gas chromatograph system coupled to a mass spectrometer with a quadrupole mass analyzer (GC/MS-7890A/5975C), equipped with a DB-5MS column (30 m × 0.25 mm ID × 0.25 μm) and a split/splitless injector manufactured by Agilent J&W made in Palo Alto, CA, USA. Ultrapure helium
Fig. 1. Procedure for the evaluation of POPs remediation in water with modified clays. 2
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Fig. 3. Response surfaces for the variables extraction time and extraction temperature using DISPME GC/MS.
(Munich, Germany). A stock solution of endosulfan at 1000.0 mg L−1 in methanol was prepared from the solid standard. A standard solution of 2,3,7,8-tetrachlorodibenzo-p-dioxin in toluene at 10.0-μg mL−1 was purchased from Sigma-Aldrich® (Bellefonte, PA, USA). Working solutions were prepared at different concentrations for each class of analyte. All extractions were carried out in 20 mL vials for SPME containing magnetic stirring.
Table 2 Analytical figures of merit for the determination of endosulfan and 2,3,7,8TCDD in aqueous samples by DI-SPME GC/MS method. Analyte Endosulfan α Endosulfan β 2,3,7,8-TCDD a
Working range (μg L−1) 0.92–25.00 0.13–25.00 0.01–0.50
R2 0.9995 0.9923 0.9187
LOD (μg L−1)
LOQ (μg L−1)
0.533 0.088 0.006
0.916 0.126 0.010
Cv (%)a 6.0 5.9 7.6
2.3. Method optimization and validation
RSD measured at the midpoint of the working range.
To optimize the SPME method, all experiments were performed with standard samples solution containing 100.00 μg L−1 of endosulfan α and endosulfan β and 1.00 μg L−1 of 2,3,7,8-tetrachlorodibenzo-pdioxin using direct immersion of the SPME fiber. The first step consisted in a univariate optimization of PDMS fiber desorption time. Desorption times of 5, 7, 10, 12, 15 and 17 min were evaluated. Then, a univariate optimization of the solvent was performed. The following solvents were evaluated: methanol, ethyl acetate, acetone, isopropanol and a mixture (1:1) of methanol-ethyl acetate. In a second step a multivariate optimization of other extraction parameters in DI-SPME mode was developed. A study was conducted using a 24 two-level full factorial design (FFD) to investigate the variables: extraction time (tex), extraction temperature (T), pH and saltingout effect by percentage of NaCl. The tests were performed in duplicate and the central point in triplicate, totaling 35 experiments. The experimental values of the variables studied are presented in Table 1. The minimum and maximum levels of the parameters were selected based on the properties of the analyzed POPs and following the levels presented in other research studies [8,10,11]. The temperature was limited by 70 °C because extraction temperatures above 70 °C cause losses of low molecular weights POPs, such as some pesticides. On the other hand, temperatures below 30 °C make it difficult to extract the POPs of higher molecular weights, like chlorinated aromatic compounds. The volume of modifier was set at 100.0 μL. The minimum level of pH was set at 4.0 and the maximum level of NaCl at 5.0% m v−1 to avoid damaging the fiber used in the extraction. The Doehlert design was applied using the most significant variables with the objective of reaching the optimum region of the investigated area. The model fits were validated using the analysis of variance (ANOVA). The p-values smaller than 0.05 were considered significant. All the statistical analyzes were performed using the statistical package Statistica 8.0 for Windows Stat soft Inc. (Tulsa, OK, USA). The method validation was developed according to Eurachem Guide “The Fitness for Purpose of Analytical Methods” of 2014 [28], evaluating
Table 3 Elemental analysis of bentonite pure and intercalated with different levels of CTA+Br− plus vermiculite pure and intercalated with different levels of CTA+Br−. Material
C (%)
H (%)
N (%)
BE B3CTA B7CTA B15CTA VE V3CTA V7CTA V15CTA
0.36 9.74 19.19 27.17 0.14 12.00 14.00 15.00
1.33 2.08 3.53 4.99 1.10 2.73 3.13 3.22
0.04 0.61 1.19 1.67 0.10 1.02 1.02 1.00
(1.5406 Å), 30 kV, 30 mA, in a range of 2θ from 4 to 70°, speed of 4° min−1, with space of 0.05 and constant time of 0.75 s per increment, equipped with polycapillary optics with parallel focus and graphite monochromator. In order to identify the functional groups present in the materials, Fourier transform infrared (FTIR) absorption spectroscopy was performed on a PerkinElmer (Massachusetts, USA) FTIR GX, in the spectral range of 400 to 4000 cm−1, resolution of 4 cm−1 and 64 scans. Samples were prepared as pellets with KBr. The lamellar structures of the materials were verified by transmission electron microscopy (TEM) measurements. Sample preparation was performed by dispersing the samples in acetone and subsequently dripping onto a carbon screen, Holey carbon-200. Transmission electron microscopy images were obtained from a G2-20-Super Twin Tecnai FEI (Hillsboro, USA) equipment. 2.2. Reagents and solutions Endosulfan α and endosulfan β were obtained from Sigma-Aldrich 3
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Fig. 4. Transmission electronic microscopic images of pure and intercalated clays materials (BE, VE, B3CTA+ and V15CTA)+.
the following performance parameters: selectivity, limit of detection, limit of quantification, working range, linearity and precision.
vermiculites (V3CTA, V7CTA, V15CTA) were studied by varying the amount of the adsorbent material per volume of aqueous solution (0.25 to 0.10 mg mL−1), fixing the contact time of the material under stirring with the aqueous solution in 10 min. Two tests were performed for each material, 12 experiments in total. Fig. 1 shows the schematic method.
2.4. Intercalation of clays The intercalation with CTA+Br− in the BE (bentonite) and VE (vermiculite) materials was performed using three CTA+Br−/clay (m m−1) ratios: 0.75, 0.35 and 0.15, following the procedure: to a suspension of CTA+Br− (100 mL of deionized water) were added 2.0 g of bentonite or vermiculite. The mixtures were left under magnetic stirring for 24 h at room temperature. The materials were then washed with deionized water to remove excess CTA+Br− and the solids were oven dried at 60 °C for 12 h [29–31]. The CTA+Br−/clay ratios defined above are theoretical. After the characterization of the materials by thermal analysis and elemental analysis, the percentage of CTA+Br− real, by mass, was obtained in the samples. The prepared materials were named using the initial letter BE or VE clay, followed by the actual surfactant content, as for example, the bentonite intercalated with 15% CTA+Br− was named B15CTA [29–31].
3. Results and discussion 3.1. Analytical method optimization The results of univariate optimizations showed that 12 min was necessary to achieve a complete desorption. Among the different matrix modifying solvents tested, the mixture MeOH-AcOEt (200 μL in 1:1 ratio) showed optimized and reproducible responses for both analytes. The parameter used to evaluate the multivariate optimization was the geometric mean of the analytes peaks areas. Analysis of the Pareto plot (Fig. 2) allowed us to evaluate the importance of each variable at a 95% confidence level. The significant variables were time and temperature of extraction. The level of significance for each regression calculated by ANOVA showed that the model was well fitted. The optimum values for each independent variable were calculated using the adjusted models to the maximum point of the function, therefore, these values should be used in the experimental procedure for the analytes extraction. The response surfaces obtained from the regression models of Doehlert design are presented in Fig. 3. The most influential factors on the response were the extraction temperature and extraction time. The
2.5. Remediation study The remediation capacity of modified bentonite and vermiculite was evaluated using standard samples solution containing 15.00 μg L−1 of endosulfan α and endosulfan β and 0.25 μg L−1 of 2,3,7,8-tetrachlorodibenzo-p-dioxin. The bentonites (B3CTA, B7CTA, B15CTA) and 4
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Fig. 5. CTA+Br− intercalation processes in bentonite and vermiculite clays for POP removal.
Fig. 6. Percent recoveries of endosulfan α, endosulfan β and 2,3,7,8-TCDD extracted from water by different bentonite/solution ratios (0.25 to 1.00 mg mL−1).
best extraction temperature was the maximum evaluated in the range of 65 to 70 °C, and the best extraction time was achieved at 30 min. It was observed that pH of the solution and the salting-out effect are not statistically significant. A slight improvement in the extraction was observed in pH values close to 6 and low concentration of NaCl. Thus, to achieve the optimum conditions for extraction of all POPs studied within the experimental domain, it was necessary to establish the temperature and extraction time at the maximum levels, 65 °C and 30 min, respectively, with no presence of NaCl and pH set at 6.0.
proposed DI-SPME method, using the optimized conditions. The results can be seen in Table 2. The analytical curve was obtained by potable water samples spiked by analytes and analyzed in triplicate for at least 6 points of uniformly spaced concentrations. A linear relationship was observed between the peak areas and analytes concentration over the full range of the calibration curves, with determination coefficients above 0.91 (p value < 0.05). The limits of detection (LOD) and quantification (LOQ) were obtained following the Eurachem guide. The method presented a good precision, low relative standard deviation (% RSD) values for all analytes were observed in a study of intra-repeatability.
3.2. Analytical method validation The principal analytical figures of merit were studied to validate the 5
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Fig. 7. Percent recoveries of endosulfan α, endosulfan β and 2,3,7,8-TCDD extracted from water by different vermiculite/solution ratios (0.25 to 1.00 mg mL−1).
3.3. Clays characterization
in the synthesis increases the interlamellar space from 1.37 nm (pure bentonite) to 1.58, 2.09 and 2.18 nm for B3CTA, B7CTA and B15CTA, respectively. [30] When CTA+Br− is added to the pure vermiculite (d001 1.14 nm), the diffraction peaks of this phase shift again to values 2θ smaller indicating that there was an interlamellar expansion of 1.24, 1.23 and 1.22 nm for V3CTA, V7CTA and V15CTA, respectively. These results also suggest low ionic exchange capacity of vermiculite, compared to bentonite, since in all three materials the amount of intercalated surfactant was similar, as well as values of interplanar distance [31].
The pure clays and the materials obtained after the intercalation [30] were characterized by different techniques. 3.3.1. Elemental analysis The elemental analysis was carried out with the purpose of estimating the CTA+Br− intercalated content in the clays, as well as to study the mass losses involved. Table 3 shows the carbon, hydrogen and nitrogen contents for the pure clays and the intercalated samples with different levels of CTA+Br−. Bentonite and vermiculite present high cation exchange capacity due to the isomorphic substitution of the major cations by lower valence cations, such as Al3+ instead of Si4+ and Mg2+ instead of Al3+. The magnitude and the location of these substitutions in the tetrahedral or octahedral sheets, produce clays with different properties. In the bentonite, these substitutions occur mainly in the octahedral sheets, that is, between the layers where there are cations that balance the excess of negative charge, consequently, the interactions between the cations and the two tetrahedral sheets are weak, leading to a great interlamellate expansion. On the other side, most substitutions occur in tetrahedral sheets for vermiculite, limiting the potential of vermiculite to expand its interlamellar space. [31]
3.3.4. Transmission electron microscopy The transmission electron microscope images of bentonite and vermiculite materials shows that after the intercalation there were not significant changes in the morphologies of the materials when compared to their pure analogy. However, the trend of cluster formation with the presence of surfactant is apparent. Already the transmission electronic microscopic images shows (Fig. 4) the lamellae present in the clays, before and after the intercalation with CTA+Br−. Transmission electron microscope measurements for bentonite (BE), vermiculite (VE), CTA+Br− pure, and for intercalated clays with different CTA+Br− contents were used to evaluate the intercalation of CTA+Br− in clays [30,31]. Fig. 5 shows the CTA+Br− intercalation processes in bentonite and vermiculite clays for POP removal.
3.3.2. Infrared spectroscopy An analysis of vibrational spectroscopy of the infrared region was carried out to identify the functional groups presents in the clays materials, pure clays and intercalated with CTA+Br−. From the CTA+Br− spectra and from the clays intercalated with it, bands in the regions of 2850–2920 cm−1 and 1600 cm−1 are observed due to the –CH and –NH vibrations, respectively. The bands at 1490 and 1475 cm−1 can be attributed to the CH3 [–N+(CH3)3] groups of the CTA+Br−, corroborating for the intercalation of surfactant molecules between the layers of the clays. For the pure and intercalated clays with CTA+Br−, there are bands in the 3500 cm−1 region due to the presence of Si–OH; and vibrations at 1160–1080 cm−1 characteristic of Si–O–Al and Si–O–Si. The bands at 650 e 450 cm−1 are assigned to the SieO group [30,31].
3.4. Remediation study Figs. 6 and 7 show the results of the evaluation of POPs remediation in aqueous samples by modified bentonite and vermiculite. The percentage of extraction of each analyte was calculated according to Eq. (1).
E(%) =
TA
BA BA
× 100
(1)
where E (%) = Extraction percentage; BA = Blank area; TA = Target analyte area of test with adsorption material. Figs. 6 and 7 present excellent results for remediation of 2,3,7,8TCDD. The dioxin was completely removed from the prepared solutions at tested ratios of material/solution. Besides, for endosulfan the extraction percentages oscillate in the range of 85.0% to 100%, observing that increasing the % CTA+ increase the extraction level. It was also
3.3.3. X-ray diffraction The composite diffractograms show that adding CTA+Br− to the pure clay, provides the mineral phase peaks change to 2θ values smaller, indicating an interlamellar expansion of the structure due to the presence of CTA+Br−. The increase of the CTA+Br− amount used 6
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observed that in general the clay materials with acronyms 7CTA and 15CTA have similar percentages of extraction, which can indicate that the materials structure was not significantly modified with higher amount of cation.
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4. Conclusions The method proposed in this study proved to be a good alternative to conventional sample preparation protocol described in the literature for the extraction of persistent organic pollutants from water. The univariate and multivariate optimizations allowed the optimization of effective conditions for the simultaneous extraction of endosulfan α, endosulfan β and 2,3,7,8-TCDD in DI-SPME mode. In addition, good results were obtained for the analytical figures of merit during validation. It has been shown that different classes of compounds, which are normally extracted under different conditions, may be extracted in a single assay by the developed method. On the other hand, modified clays were tested to evaluate their efficiency as a remediation material for POPs in water, and all materials tested showed excellent results, reaching for endosulfan α, endosulfan β and 2,3,7,8-TCDD extraction percentages > 85.0% in the studied conditions. Thus, these modified clays establish a greater advantage for the remediation of aqueous samples according to low cost and physicochemical properties that allow a greater adsorption of analytes. Acknowledgement The authors thanks the financial support of the Brazilian Institutions: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Ministério da Saúde. References [1] Stockholm Convention on Persistent Organic Pollutants (POPs), Text and Annexes; The Secretariat of the Stockholm Convention, 2, 3 (2010), pp. 33–56. [2] L. Ritter, K. Solomon, J. Forget, Persistent organic pollutants: an assessment report on DDT, aldrin, dieldrin, endrin, chlordane, heptachlor, hexachlorobenzene, mirex, toxaphene, PCBs, dioxins and furans, Report for the International Programme on Chemical Safety (IPCS) Within the Framework of the Inter-Organization Programme for the Sound Management of Chemicals (IOMC), 2005, pp. 7–18. [3] US EPA, Reregistration Eligibility Decision for Endosulfan; EPA-738-R-02-013, (2002), pp. 4–8. [4] ATSDR, Toxicological Profile for Chlorinated Dibenzo-p-dioxins, vols. 1-8, Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA, 1998, pp. 19–56. [5] J. Saurat, O. Sorg, et al., The Cutaneous lesions of dioxin exposure: lessons from the poisoning of Victor Yushchenko, Toxicol. Sci. 125 (2012) 310–317. [6] U.S. EPA, 2,3,7,8-Tetrachlorodibenzo-p-dioxin (2,3,7,8,-TCDD) Summary, Office of Research and Development, Cincinnati, OH, https://www.epa.gov/sites/ production/files/2016-09/documents/2-3-7-8-tetrachlorodibenzo-p-dioxin.pdf, (2000) , Accessed date: 26 April 2019. [7] M. Shamsipur, N. Yazdanfar, M. Ghambarian, Combination of solid-phase extraction with dispersive liquid–liquid microextraction followed by GC–MS for determination of pesticide residues from water, milk, honey and fruit juice, Food Chem. 204 (2016) 289–297.
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