Chemosphere 90 (2013) 657–664
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Pyrene photochemical species in commercial clays Silvio Cesar Godinho Teixeira a, Anabela Oliveira b,c, Paulo Duarte b, Luis Filipe Vieira Ferreira b, Josino Costa Moreira d, Daniel Vidal Peréz e, Monica Regina da Costa Marques a,⇑ a
Instituto de Química, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier, 524, Maracanã, 20550-900 Rio de Janeiro, RJ, Brazil Centro de Química-Física Molecular, and IN-Institute of Nanoscience and Nanotechnology, Complexo Interdisciplinar, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c Centro Interdisciplinar de Investigação e Inovação, Escola Superior de Tecnologia e Gestão, Instituto Politécnico de Portalegre, Lugar da Abadessa, Apartado 148, 7300-901 Portalegre, Portugal d Centro de Estudos em Saúde do Trabalhador e Ecologia Humana, Fundação Oswaldo Cruz, Avenida Brasil, 4365, Manguinhos, 21040-360 Rio de Janeiro, RJ, Brazil e Centro Nacional de Pesquisa de Solos, Empresa Brasileira de Pesquisa Agropecuária, Jardim Botânico, 1029, Jardim Botânico, 22460-000 Rio de Janeiro, RJ, Brazil b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Pyrene photochemistry on three
excimers from pyrene were detected. " Excimer formation is function of the amount Pyrene of adsorbed on the clay. " Emission from pyrene crystals was detected for high loading samples. " Lifetime distribution analysis identified all pyrene emissions and their lifetimes.
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. Kaolinite Acid bentonite Montmorillonite
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Article history: Received 2 May 2012 Received in revised form 15 August 2012 Accepted 8 September 2012 Available online 22 October 2012 Keywords: Clay Pyrene Excimer emissions Sorption
a b s t r a c t The photochemistry of pyrene, a polycyclic aromatic hydrocarbon, adsorbed on kaolinite, sodium montmorillonite and acid bentonite K10Ò was investigated to determine how the concentration and structure of the clay minerals affect the formation of different species of pyrene. Fluorescence emission spectra were obtained using reflection geometry for pyrene at the concentrations ranging from 0.5 to 100.0 lmol g1 (pyrene:clay). Two pyrene photochemical species were observed, the monomer, which fluoresces at 396 nm, and its excimer which fluoresces at 470 nm. The formation of excimers occurred first on the kaolinite, due to the smaller surface area. In the acid montmorillonite, the fixed interlamellar space provided greater specific area, leading to lower formation of excimers. Emission from pyrene crystals was also detected for samples with high loadings. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Environmental problems caused by spillages, discharges or accidents involving fossil fuels and their derivatives are common.
⇑ Corresponding author. Tel.: +55 21 2334 0563; fax: +55 21 2334 0159. E-mail address:
[email protected] (M.R.C. Marques). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.09.037
It is of particular interest the determination of the comportment of these substances in the soil and their destination once reaching it (Azevedo et al., 2007). Among the fuel components, polycyclic aromatic hydrocarbons (PAHs) have received a great deal of attention due to their carcinogenic potential (ATSDR, 1995) and their significant environmental impact (Rolle et al., 2011). The soil acts as a filter, as a function of its adsorption capacity, both retaining the nutrients required by plants and immobilizing
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pollutants, so reducing their movement and leaching towards sensitive areas (ex. water bodies) certainly contributes to diminishing their availability in the food chain (Chiou, 2002; Young and Weber, 2005; Macht et al., 2011; Bertagnolli et al., 2011). Among the soil components responsible for this adsorption, the action of the organic component stands out, such that most studies correct for the level of organic matter present in the soil (KOC or KOM) when calculating the partition constant (KD) Schwarzenback et al., 2003; Grotenhuis and Rijnaarts, 2011. However, under certain circumstances, the clay fraction is observed to participate significantly as the sorption power of the soil increases, notably when there is a low level of organic matter in the soil (Hwang et al., 2003; Zhu et al., 2004; Maliszewska-Kordybach, 2005). Important differences between soils from the tropical and cold or temperate region must also be considered. Among the differences, the soil of the tropics are more acidic and have fewer weathered minerals because the 2:1 mineral transformation, such as of smectite and micas into kaolinites with low crystallinity (Schaefer et al., 2008). Pyrene, a polycyclic aromatic hydrocarbon, is a very versatile fluorescent probe. Frampton et al. (2000) studied the structure of pyrene at low temperature and determined that the c plane (the height within the crystalline cell) of the pyrene molecule is 8.260 Å. Pyrene can form excimers. X-ray diffraction and Raman studies have shown that there are two possible conformations: face-to-face or stacking geometry, where the rings directly overlay each other and are sometimes shifted; and perpendicular geometry, where one molecule is perpendicular to the other. The pyrene excimer molecules are separated by 3.44–3.56 Å and the axis may by dislocated by 0.78 Å (Zallen et al., 1976; Jones et al., 1978). The fluorescence emission spectrum of pyrene in hydrophobic solvents is characterized by the presence of monomers that have three very characteristic vibronic bands. These bands appear at 372, 384, 391 and 412 nm and correspond to the vibronic modes of the S1 ? S0 emission (Mohanambe and Vasudevan, 2006). In turn, studies to characterize the species of pyrene adsorbed on different solid surfaces have shown the existence of two photochemical species: the monomeric species, with maximum emission at 396 nm and its excimer (pyrene dimer in the excited state) at 470 nm (Bauer et al., 1983; Oliveira et al., 2004). In a study of the emission of pyrene adsorbed onto synthetic expansive clay, laponite, Labbé and Reverdy (1988) observed that increasing the concentration of pyrene led to the formation of excimers. After lyophilization of the clay sample containing the pyrene excimer species, the authors observed that the intensity of the excimer decreased and that the emission of the vibronic bands for the monomer increased. However, this phenomenon was reversible. That is, adding water caused the clay to swell and the excimer species to regenerate. According to the authors, this fact is attributable to the presence of water in the interlamellar space, which increases the polarity of the medium, repelling the pyrene molecules and generating the excimer. A similar result was obtained by Cione et al. (1998) when studying the emission of naphthalene in synthetic expansive clays: a montmorillonite and a laponite. The authors observed that as the montmorillonite is an expansive clay with a greater degree of hydration reason why the emission intensity of the excimer was greater than that observed for the laponite. The objective of this study is to elucidate the behavior of pyrene in clay minerals extracted from Brazilian deposits (kaolinite and montmorillonite), which are representative of the minerals found in tropical soils, and to compare them extracted with a commercial expansive acidic clay (bentonite K-10Ò).
2. Experimental 2.1. Materials Commercial sodium montmorillonite and kaolinite clays were supplied by Bentonit, BrasilÒ and acid bentonite K10Ò was purchased from Aldrich. Pyrene was purchase from Aldrich, in the highest purity available (99%) and was used as received. All the solvents used for sample preparation were from also from Aldrich, Chromasolv Plus grade. 2.2. Characterization of clays The surface specific area of the clay were determined adsorption–desorption of nitrogen at 77 K using Micromeritics A.S.A.P. 2000, using BET equation and the external surface area was determined by BJH equation. 2.3. Pre-treatments for X-ray diffraction identification of mineralogical composition of the commercial clays The commercial clay samples (50 g) had the organic matter removed by sodium pyrophosphate and were then treated with dithionite–citrate–bicarbonate to extract the iron (Mehra and Jackson, 1960; S´rodon´, 2009). They were then dispersed by vigorously shaking (±12 000 rpm) in 250 cm3 of a 1 M L1 NaOH solution. After 15 min, the dispersion was sieved with a 0.053 mm mesh, dried at 40 °C and then analyzed by X-ray diffraction (EmbrapaEmpresa Brasileira de Pesquisa Agropecuária, 1999; Jackson, 2005; Calderano et al., 2009). 2.4. Preparation of the oriented slides for X-ray diffraction A 3 g sample of clay was placed in a centrifuge tube (model Supra 22 K, Hanil) with 40 cm3 of distilled water. The mixture was sonicated (model Sonifier 250, Branson) for 3 min to fully disperse the clays (Macht et al., 2011; Silva et al., 2012). After centrifuging at 18 000 rpm, approximately 1 g of the sample was transferred to a glass microscope slide and wetted until it formed a homogeneous paste. Another glass slide was used to smear the sample to orientate the clay particles. Then the slide was dried at room temperature and analyzed by (total) X-ray diffraction. However, in order to allow the differentiation and identification of the clays, due to effects on the basic spacing, some treatments were performed: the solvation with ethylene glycol, heating treatments at various temperatures, and cation saturation with Ca, Mg and K to minimize the fluctuations of water between the layers of the crystal (Brindley and Brown, 1980). In that context, 1 g of the material was saturated with 1 M dm3 CaCl2 and another 1 g with 1 dm3 KCl. These were then washed with double distilled water to remove the excess ions and new oriented slides were prepared (Omotoso and Mikula, 2004; Senna et al., 2008). Diffractograms were obtained for clay samples: natural, dried at room temperature, (total); saturated with Mg to differentiate expansive 2:1 minerals (vermiculite and smectite) from the non-expansive (micas) mineral (Mg); saturated with Mg, followed by solvation with ethylene glycol to differentiate vermiculite from smectite (Mg Glycol); saturated with K to differentiate chlorite and vermiculite (VHE) and smectite (EHE) with Al-hydroxyl interlayed with expansive 2:1 minerals (K) and saturated with K and heated to 110, 350 and 550 °C to differentiate chlorite from VHE and EHE (K 110, 350 and 550 °C) (Bortoluzzi et al., 2008; Melo et al., 2009). The minerals were identified from the peaks corresponding to the h0 0 1i plane, following Brindley and Brown (1980). X-ray diffractograms were obtained using a Rigaku diffractometer, model mini-
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flex II, with Ka copper radiation (k = 1.54 Å), monitoring the diffraction 2H degrees steps size from 2° to 45°, 2 s signal accumulation time, 40 kV voltage and 30 mA current. 2.5. Study of the expansion caused to the interlamellar space by different solvents Oriented slides of montmorillonite were prepared for X-ray diffraction. The diffractograms were obtained under the following conditions: (i) totally dry montmorillonite, (ii) montmorillonite wetted with ethanol (iii) montmorillonite wetted with dichloromethane (DCM). 2.6. Pyrene impregnation into the clay samples The adsorption of pyrene following the solvent (ethanol) slowly evaporation is a well known process, which leads to samples identical to ‘‘equilibrium’’ samples, the ones where the host in the form of powder is put in contact with the guest solution until an equilibrium is reached, for low loadings of the guest. For the high loadings, some pools of microcrystals may be formed, as was verified in many other case studies, e.g. Oliveira et al. (2004); Branco et al. (2005); Vieira Ferreira and Ferreira Machado (2007). A 1 g sample of each clay was wetted with 25 dm3 of ethanol in a beaker and the phases were stirred by hand. Different volumes of a solution of pyrene were added to the clays so that the final concentrations of the analyte adsorbed on the clay after the solvent was evaporated were 0.5, 5.0, 25.0 and 100.0 lmol g1 (pyrene:clay). These mixtures were stirred until the solvent had completely evaporated. To ensure that the pyrene was uniformly distributed over the entire surface area of the clay mineral, two new aliquots of 25 cm3 of ethanol were added to the clays with deposited pyrene and solvent was again evaporated. 2.7. Laser Induced Luminescence (LIL) and Lifetime Distribution Analysis (LDA) The clay mineral samples contaminated with pyrene were analyzed in 5 mm light path quartz cuvettes. A description of the LIL system was presented recently in reference Vieira Ferreira and Ferreira Machado (2007). For the laser-induced luminescence experiments, a N2 laser (PTI model 2000, ca. 600 ps fwhm, 1.0 mJ pulse1) and a reflection geometry mode were employed. The light arising from the solid powdered samples after excitation by the laser pulse was collected by a collimating beam probe coupled to an optical fiber (fused silica) and detected by a gated intensified charge coupled device (Andor ICCD detector, i-Star 720). The ICCD was coupled to a fixed imaging compact spectrograph (Shemrock 163). The system could be used either by capturing all light emitted by the sample or in a time-resolved mode by the use of the internal delay capability of the i-Star 720. The ICCD has high-speed gating electronics (2.3 ns) and intensifier and covers the 200–950 nm wavelength range. Time-resolved emission spectra were available in the nanosecond to second time range both in transmission or diffuse reflectance modes. Recently we developed an important tool for a lifetime distributions analysis (LDA) of emissions of probes adsorbed onto heterogeneous surfaces (Branco et al., 2005). This new methodology allows for asymmetric distributions and uses pseudo-Voigt profiles (Gaussian and Lorentzian mixture) instead of pure Gaussian or Lorentzian distributions. A very simple and widely available tool for fitting has been used, the Microsoft Excel Solver. This is a very convenient way to treat the emission or transient absorption decay data because that reflects the multiplicity of sites available for the probe onto the specific surface under study.
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In this model, it is assumed that the lifetime’s distribution of an excited probe adsorbed on a heterogeneous, porous substrate is a consequence of a distribution of DG° for the probe adsorption on the substrate around a mean value. This model allows one to obtain a distribution of lifetimes, a, which reflects the decay of the emissive species in different adsorption sites, where a is the relative weight in the total distribution. The use of a sum of several exponentials to analyze the decay of probes onto heterogeneous surfaces is a description without physical meaning. LDA is a much better tool to study decays of a probe on heterogeneous surfaces. However, the problem of recovering the distribution of lifetimes, a, from the decay curve is an inverse problem like many others in physical science, and its illconditioned nature is well-known and largely discussed in the literature. 3. Results 3.1. Mineralogical characterization The diffractogram obtained with the clay mineral kaolinite is given in Fig. 1a. The clay mineral kaolinite predominates in the commercial kaolinite, shown by the diffraction peak at 12.45 (2°H) – 7.14 Å (Fig. 1a). Kaolinite degradation was observed only in the sample doped with K heated to 550 °C (Fig. 1a – K 550 °C) (Gomes, 1988; Silva et al., 2012). This sample did not appear to be contaminated by expansive clay once the diffractogram did not show diffraction peaks in the region 2–10 (2°H), which relates to a diffraction from 44 to 11 Å. The diffractogram obtained with the clay mineral montmorillonite is given in Fig. 1b. In these diffractograms, the structure of the clay mineral montmorillonite is confirmed by the diffraction peak at 5.85 (2°H) or 15.13 Å. The saturation treatment with Mg and glycol indicates that the interlamellar space expanded from 15.13° to 5.15 (2°H) or 17.19 Å, as expected for a montmorillonite. The bentonite K10Ò sample gave a diffraction peak at 5.5 (2°H) corresponding to an interlamellar space of 16.06 Å. The diffraction peak for the sample subjected to saturation with K moved to 6.7 (2°H) or 13.19 Å. After the slide had been heat treated, the subsequent diffractograms suggested that the clay mineral had degraded as the diffraction peak had disappeared, which agrees with the literature for montmorillonite. The saturation treatment with Mg and glycol showed that the interlamellar space expanded from 16.06 to 16.67 Å. This small expansion agrees with the behavior expected for a montmorillonite subjected to acid treatment, which leads to the space becoming fixed, inhibiting its expansion. 3.2. Swelling study As this study has the objective of evaluating the photochemical behavior of pyrene adsorbed onto clays, the first phase of the work was to investigate the expansibility of the interlamellar region of the clay montmorillonite under the action of ethanol and dichloromethane, solvents in which the pyrene is more soluble than in water (130 lg L1). To this end, X-ray diffractograms were obtained for montmorillonite wetted with ethanol and dichloromethane (DCM) (Fig. 2). In the diffractogram obtained for the dry montmorillonite it was possible to verify that the size of the clay interlamellar space was 14.73 Å. In the samples wetted with ethanol and with dichloromethane, the interlamellar space changed from 14.73 Å to, respectively, 18.64 Å and 19.04 Å. These results prove that in addition to water, these organic solvents cause the interlamellar space to expand, and may be used to solubilise the pyrene in adsorption studies in clays. Ethanol was chosen based on the greater solubility of pyrene in ethanol than in water and
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Fig. 1. X-ray diffractograms of the oriented slides of the clay minerals extracted from the commercial samples of (a) kaolinite, (b) montmorillonite and (c) acid bentonite (1 – pure clay, 2 – iron-free sample saturated with potassium, 3 – iron-free sample saturated with potassium and heat treated at 110 °C, 4 – iron-free sample saturated with potassium and heat treated at 350 °C, 5 – iron-free sample saturated with potassium and heat treated at 550 °C, 6 – clay saturated with magnesium and 7 – clays saturated with Mg and glycolated).
the lower volatility of ethanol in comparison to DCM. This enabled the use of higher concentrations of pyrene and, increasing the analyte’s contact time with the clays, which ensures better homogenisation of the samples.
3.3. Photochemical studies Initially, the clays were assessed by laser induced fluorescence spectroscopy. Fig. 3 shows the fluorescence emission spectra of
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Fig. 2. Expansion of the interlamellar space of dry montmorillonite and of EtOH(EtOH-wetted montmorillonite) and dichloromethane-wetted (DCM-wetted montmorillonite) montmorillonite.
the clay minerals before the addition of pyrene, obtained with excitation at 337 nm and emission from 200 to 800 nm, using reflection geometry. With the kaolinite and bentonite, fluorescence had maxima in the region of 406 nm. We found that such clays presented fluorescent organic compounds, derived from the residue of recalcitrant organic matter at the end of the process of fractionation and extraction from the commercial product (Fig. 3).
Fig. 3. Fluorescence emission spectra for the residue of organic matter present in the (a) kaolinite; (b) acid bentonite and (c) montmorillonite.
Fig. 4(1) shows the normalised fluorescence emission spectra for pyrene at a concentration of 0.5 lmol g1 (pyrene:clay). We observed that for 0.5 lmol g1 the hydrocarbon fluoresces as a monomer in all of the clays where it was adsorbed, suggesting that the analyte appeared mainly to have been adsorbed in a single layer. We can also see from the shoulder at 450 nm that there was a small formation of excimers in the kaolinite and montmorillonite.
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Fig. 4. Normalised fluorescence emission spectra for pyrene obtained in the presence of (a) kaolinite, (b) acid bentonite and (c) montmorillonite at different concentrations of pyrene:clay: (1) 0.5 lmol g1. (2) 5 lmol g1. (3) 25 lmol g1. (4) 100 lmol g1.
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Table 1 Surface area of clays.
a b
Clay
External surface areaa (m2 g1)
Specific surface areab (m2 g1)
Kaolinite Acid Bentonite Montmorillonite
15 238 58
15 254 93
Table 3 Mean lifetimes and percentages obtained for each distribution for 25 lmol g1 recovered from time resolved luminescence decays observed at 420 nm. Crystals
Kaolinite Acid bentonite Montmorillonite
BET equation of ASAP. BJH equation of ASAP.
Table 2 Mean lifetimes and percentages obtained for each distribution for 0.5 lmol g1 recovered from time resolved luminescence decays observed at 420 nm. Crystals
Kaolinite Acid bentonite Montmorillonite
Monomer
Excimer
Lifetime (ns)
%
Lifetime (ns)
%
Lifetime (ns)
%
– – –
– – –
80 102 73
45.7 84.2 80.9
21 22 17
54.3 15.8 19.1
Fig. 4(2) shows the normalised fluorescence emission spectra for pyrene at a concentration of 5 lmol g1 (pyrene:clay). In the kaolinite, the pyrene fluoresces partly as a monomer and partly as a excimer. In the montmorillonite and acid bentonite, the pyrene emits mainly as a monomer. These data reveals that increasing the concentration of the pyrene led to the formation of multiple layers of analyte adsorbed onto the kaolinite. This suggests that the quantity of adsorption sites capable of holding the hydrocarbon in a single layer is lower in the kaolinite when compared with the quantity of sites found in the other clays studied (Fig. 4(1)). Labbé and Reverdy (1988) compared the specific area (750 m2 g1) of a type 2:1 mineral as laponite with its external surface area (350 m2 g1) determined by the BET method and concluded that
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55.5 19.5 24.5
– – 83
– – 29.7
20 27 20
44.5 80.5 45.8
the specific area of the mineral had three regions: the area of the edges, the external surface area and the interlamellar space. Bertagnolli et al. (2011) studied a bentonite and also verified differences between the specific area (68.23 m2 g1), measured by the EGME method, and the surface area (25.34 m2 g1), measured by the BET method, corroborating the suggestion of Labbé and Reverdy (1988). Macht et al. (2011) verified that the montmorillonites are the clays with the largest specific area of all of the clay minerals, exactly because of the capacity of the interlamellar region to expand when hydrated. The specific areas of sodium montmorillonite and bentonite K10Ò were higher that external surface area (Table 1). This fact is not observed to kaolinite. As the type 1:1 minerals is predominant in the kaolinite, while the others are predominantly 2:1, this result suggest that the interlamellar space was the region that most contributed to the large specific area of the mineral. The results obtained at the concentration of 25 lmol g1 (pyrene:clay) are given in Fig. 4(3). In the kaolinite, the main formation was of excimers of pyrene, resulting from the adsorption of the analyte in multiple layers. In the montmorillonite, the pyrene fluoresces partly as a monomer and partly as an excimer. Whereas, with the bentonite, only monomer emission was observed. We propose that the formation of excimers in the kaolinite resulted from its smaller specific area. For the montmorillonite, the proposed hypothesis is that the formation of excimers occurred as a
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Fig. 5. Lifetime distribution for pyrene 0.5 lmol g1 on: (a) kaolinite; (b) acid bentonite and (c) montmorillonite and pyrene 25 lmol g1 on: (d) kaolinite; (e) acid bentonite and (f) montmorillonite, recovered from time resolved luminescence decays observed at 420 nm.
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function of the increase in concentration of pyrene in a limited space that was still polarised due to the water molecules hydrating the cations in the interlamellar region. The water repelled the pyrene molecules, which formed the excimer when they got close together (Labbé and Reverdy, 1988; Minquan et al., 1996; Cione et al., 1998; Mohanambe and Vasudevan, 2006). Finally, for the bentonite, the acid treatment of the expansive clay minerals has the objective of replacing the cations (Na+, K+, Ca2+ and Mg2+) present in the interlamellar space with H+, which fixes this space. Hence, the interlamellar region did not show great size variation in the expansibility tests using the pure samples and the samples saturated with Mg and ethylene glycol analysed by X-ray diffraction (Fig. 1c). This fact suggests that a single layer may diffuse with the interlamellar space fixed, as shown by the spectrum for the emission of monomers of pyrene. The results obtained at the concentration of 100 lmol g1 (pyrene:soil) are given in Fig. 4(4). The pyrene remained in the excimer form on the kaolinite. In the montmorillonite, it was demonstrated that the main formation was of excimers, while in the bentonite, the pyrene remained as a monomer. These results support the hypotheses proposed above. In order to fully understand the origin of these emissions, a lifetime distributions analysis (LDA) was performed as described in reference Vieira Ferreira and Ferreira Machado (2007). This analysis is needed for studying pyrene included within the different samples since these samples have different conformations and even different possible adsorption sites. Therefore continuous distributions of lifetimes provide a better description than the assumption of simple discrete lifetimes. LDA was performed at three different wavelengths 298, 420 and 449 nm. Tables 1 and 2 presents the mean lifetimes and percentages obtained for each distribution for 0.5 lmol g1 and 25 lmol g1 respectively. For 0.5 M g1 concentration, all samples show the presence of monomers of pyrene and the presence of excimer. Higher monomer lifetime in acid bentonite is probably due to the tighter conditions of pyrene inside this host. The excimer lifetimes are less affected by the size of the cavities because his formation process already implies a certain mobility of the pyrene molecules to achieve a specific excimer formation. For 25 lmol g1 concentration, all samples have a very small lifetime (4–7 ns) that can be attributed to the formation of crystals due to the high concentration of pyrene (Table 3). Only sample montmorillonite have monomer, but the monomeric form is present in all samples in less extend (results of LDA performed at 398 nm not shown). Formation of more excimer was expected because there are more pyrene molecules and so there is a higher interaction between them, and as a consequence of that more formation of excimer. Fig. 5 shows the lifetime distribution recovered from time resolved luminescence decays observed at 420 nm, for 0.5 lmol g1 and 25 lmol g1 of pyrene, respectively. The results of lifetime distribution analysis are in agreement with the hypotheses proposed above.
4. Conclusions The excimer formation of pyrene was a function of its concentration adsorbed in the clay material, the specific area of the mineral and the presence and conditions of the interlamellar space and it occurred for the second lowest concentration of pyrene used in the case of the kaolinite, the clay mineral with lowest specific area. In the minerals with larger surfaces, such as the montmorillonites, the presence of the expansible interlamellar space slowed the excimer formation. In the acid montmorillonite (bentonite K10), whose interlamellar space was fixed, the monomeric fraction dominated, even with the highest concentration of pyrene used in this study.
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While in the montmorillonite, despite its larger interlamellar space, the increase in concentration of the analyte associated with the polarity of the medium forced the molecules closer, leading to the formation of the excimer. Acknowledgements We thank the Rio de Janeiro State Research Foundation (FAPERJ) and the National Foundations (CAPES and CNPq – INOG-UERJ) for financial support. References ATSDR (Agency for Toxic Substances and Disease Registry), 1995. Toxicological Profile for Polycyclic Aromatic Hydrocarbons (PAHs), US Department of Health & Human Services, Agency for Toxic Substances and Disease Registry, Atlanta, 458p . Azevedo, C.A., Pedron, F.A., Dalmolin, R.S.D., 2007. A evolução da vida e as funções do solo no ambiente. In: Tópicos em ciências do solo, V. Sociedade Brasileira de Ciências do Solo, Viçosa, pp. 1–48. Bauer, R.K., De Mayo, P., Okada, K., Ware, W.R., Wu, K.C., 1983. Surface photochemistry, the effects of coadsorbed molecules on pyrene luminescence and acenaphthylene dimerization on silica gel. J. Phys. Chem. 87, 460–466. Bertagnolli, C., Kleinübing, S.J., Silva, M.G.C., 2011. Preparation and characterization of a Brazilian bentonite clay for removal of copper in porous beds. Appl. Clay Sci. 53, 73–79. Bortoluzzi, E.C., Pernes, M., Tessier, D., 2008. Mineralogia de partículas envolvidas na formação de gradiente textural em um Argissolo subtropical. R. Bras. Ci. Solo. 32, 997–1007. Branco, T.J.F., Botelho do Rego, A.M., Ferreira Machado, I., Vieira Ferreira, L.F., 2005. A luminescence lifetime distributions analysis in heterogeneous systems by the use of Excel’s Solver. J. Phys. Chem. B 109, 15958–15967. Brindley, G.W., Brown, G., 1980. Crystal Structures of Clays Minerals and their X-ray Identification. Mineralogical Society, London. Calderano, S.B., Duarte, M.N., Gregoris, G., 2009. Análise mineralógica das frações finas do solo por difratometria de raios-X. Comunicado técnico 53, 1–9. Chiou, C.T., 2002. Partition and Adsorption of Organic Contaminants in Environmental Systems. John Wiley, New Jersey. Cione, A.P.P., Scaiano, J.C., Neumann, M.G., Gessner, F., 1998. The excimer emission of aromatic hydrocarbons on clays. J. Photochem. Photobiol., A 118, 205–209. Embrapa-Empresa Brasileira de Pesquisa Agropecuária, 1999. Manual de métodos de análise de solo. Embrapa-Serviço Nacional de Levantamento e Conservação de Solos, Rio de Janeiro. Frampton, C.S., Knight, K.S., Shankland, N., Shankland, K., 2000. Single-crystal X-ray diffraction analysis of pyrene II at 93 K. J. Mol. Struct. 520, 29–32. Gomes, C.F., 1988. Argilas, o que são e para que servem. Fundação Calouste Gulbenkian, Lisboa. Grotenhuis, T.J.T.C., Rijnaarts, H.H.H.M., 2011. In situ remediation technologies. In: Swartjes, F.A. (Ed.), Dealing with Contaminated Sites. Springer, Dordrecht, pp. 949–977. Hwang, S., Ramirez, N., Cutright, T.J., Ju, L.K., 2003. The role of soil properties in pyrene sorption and desorption. Water. Air. Soil Pollut. 143, 65–80. Jackson, M.L., 2005. Soil Chemical Analysis – Advanced Course. Parallel Press, Madison. Jones, W., Ramda, S., Thomas, J.M., 1978. Novel approach to the determination of the crystal structures of organic molecular crystals: low temperature form of pyrene. Chem. Phys. Lett. 54, 491–493. Labbé, P., Reverdy, G., 1988. Adsorption characteristics of polycyclic aromatic compounds on clay: pyrene as a photophysical probe on laponite. Langmuir 4, 419–425. Macht, F., Eusterhues, K., Pronk, G.J., Totsche, K.U., 2011. Specific surface area of clay minerals: comparison between atomic force microscopy measurements and bulk-gas (N2) and -liquid (EGME) adsorption methods. Appl. Clay Sci. 53, 20–26. Maliszewska-Kordybach, B., 2005. Dissipation of polycyclic aromatic hydrocarbons in freshly contaminated soils—the effect of soil physicochemical properties and aging. Water. Air. Soil Pollut. 168, 113–128. Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soils and clays by dithionite–citrate system buffered with sodium bicarbonate. Clay Miner. 7, 317–327. Melo, V.F., Mattos, J.M.S.M., Lima, V.C., 2009. Métodos de concentração de minerais 2:1 secundários na fração argila visando sua identificação por difratometria de raios-X. Rev. Bras. Ciênc. Solo. 33, 527–539. Minquan, W., Guodong, Q., Mang, W., Xianping, F., Zhanglian, H., 1996. The influence of the matrix on the fluorescence properties of pyrene. Mater. Sci. Eng. B40, 67–71. Mohanambe, L., Vasudevan, S., 2006. Aromatic molecules in restricted geometries: pyrene excimer formation in an anchored bilayer. J. Phys. Chem. B 110 (29), 14345–14354. Oliveira, A.S., Vieira Ferreira, L.F., Silva, J.P., Moreira, J.C., 2004. Surface photochemistry: photodegradation study of pyrene adsorbed onto microcrystalline cellulose and silica. Int. J. Photoenergy 6, 205–213.
664
S.C.G. Teixeira et al. / Chemosphere 90 (2013) 657–664
Omotoso, O.E., Mikula, R.J., 2004. High surface areas caused by smectitic interstratification of kaolinite and illite in Athabasca oil sands. Appl. Clay Sci. 25, 37–47. Rolle, M., Maier, U., Grathwohl, P., 2011. Contaminant fate and reactive transport in groundwater. In: Swartjes, F.A. (Ed.), Dealing with Contaminated Sites. Springer, Dordrecht, pp. 851–885. Schaefer, C.E.G.R., Fabris, J.D., Ker, J.C., 2008. Minerals in the clay fraction of Brazilian Latosols (Oxisols): a review. Clay Miner. 43, 137–154. Schwarzenback, R.P., Gschwend, P.M., Imboden, D.M., 2003. Environmental Organic Chemistry, second ed. John Wiley, New Jersey. pp. 275–330. Senna, J.A., Souza Filho, C.R., Angélica, R.S., 2008. Characterization of clays used in the ceramic manufacturing industry by reflectance spectroscopy: an experiment in the São Simão ball-clay deposit. Braz. Appl. Clay Sci. 41, 85–98. Silva, L.A., Teixeira, S.C.G., Pérez, D.V., Marques, M.R.C., 2012. Impact of Chemical Oxidation on Brazilian Soils. J. Braz. Chem. Soc., 1–5.
S´rodon´, J., 2009. Quantification of illite and smectite and their layer charges in sandstones and shales from shallow burial depth. Clay Miner. 44, 421–434. Vieira Ferreira, L.F., Ferreira Machado, I.L., 2007. Surface photochemistry: organic molecules within nanocavities of Calixarenes. Curr. Drug Discovery Technol. 4, 229–245. Young, T.M., Weber Jr., W.J., 2005. Sorption and desorption rates for neutral organic compounds in soils. In: Dick, W.A. (Ed.), Chemical Processes in Soils, Soil Science Society of America, Madison, pp. 519–561. Zallen, R., Griffiths, C.H., Slade, M.L., Hayek, M., Brafman, O., 1976. The solid state transitions in pyrene. Chem. Phys. Lett. 39, 85–89. Zhu, D., Herbert, B.E., Schlautman, M.A., Carraway, E.R., Hur, J., 2004. Cation–P bonding: a new perspective on the sorption of polycyclic aromatic hydrocarbons to mineral surfaces. J. Environ. Qual. 33, 1322–1330.
