This article was downloaded by: [Tulane University] On: 04 September 2013, At: 13:41 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Analytical Letters Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lanl20 Sorption and Preconcentration of the Herbicides Atrazine, Simazine, and Ametryne on Montmorillonite Edivaltrys Inayve Pissinati de Rezende a , Patricio Guillermo Peralta- Zamora a , Wilson de Figueiredo Jardim b , Cristiane Vidal b & Gilberto Abate a a Departamento de Química, Universidade Federal do Paraná, Curitiba, Paraná, Brazil b Instituto de Química, Universidade Estadual de Campinas, Campinas, São Paulo, Brazil Accepted author version posted online: 04 Sep 2012.Published online: 22 Jan 2013. To cite this article: Edivaltrys Inayve Pissinati de Rezende , Patricio Guillermo Peralta-Zamora , Wilson de Figueiredo Jardim , Cristiane Vidal & Gilberto Abate (2013) Sorption and Preconcentration of the Herbicides Atrazine, Simazine, and Ametryne on Montmorillonite, Analytical Letters, 46:3, 439-451, DOI: 10.1080/00032719.2012.725191 To link to this article: http://dx.doi.org/10.1080/00032719.2012.725191 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
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This article was downloaded by: [Tulane University]On: 04 September 2013, At: 13:41Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Analytical LettersPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lanl20
Sorption and Preconcentration of theHerbicides Atrazine, Simazine, andAmetryne on MontmorilloniteEdivaltrys Inayve Pissinati de Rezende a , Patricio Guillermo Peralta-Zamora a , Wilson de Figueiredo Jardim b , Cristiane Vidal b &Gilberto Abate aa Departamento de Química, Universidade Federal do Paraná,Curitiba, Paraná, Brazilb Instituto de Química, Universidade Estadual de Campinas,Campinas, São Paulo, BrazilAccepted author version posted online: 04 Sep 2012.Publishedonline: 22 Jan 2013.
To cite this article: Edivaltrys Inayve Pissinati de Rezende , Patricio Guillermo Peralta-Zamora ,Wilson de Figueiredo Jardim , Cristiane Vidal & Gilberto Abate (2013) Sorption and Preconcentrationof the Herbicides Atrazine, Simazine, and Ametryne on Montmorillonite, Analytical Letters, 46:3,439-451, DOI: 10.1080/00032719.2012.725191
To link to this article: http://dx.doi.org/10.1080/00032719.2012.725191
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.
This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
SORPTION AND PRECONCENTRATION OF THEHERBICIDES ATRAZINE, SIMAZINE, ANDAMETRYNE ON MONTMORILLONITE
Edivaltrys Inayve Pissinati de Rezende,1 Patricio GuillermoPeralta-Zamora,1 Wilson de Figueiredo Jardim,2
Cristiane Vidal,2 and Gilberto Abate11Departamento de Quımica, Universidade Federal do Parana, Curitiba,Parana, Brazil2Instituto de Quımica, Universidade Estadual de Campinas, Campinas,Sao Paulo, Brazil
This paper describes the treatment of montmorillonite (MT), with Kþ (MTK), Naþ
(MTNa), and Ca2þ (MTCa) to explore the use of these minerals for the extraction and
preconcentration of the herbicides atrazine, simazine, and ametryne from aqueous medium.
In the sorption process, the three materials exhibited good performance; ametryne was
totally sorbed. For atrazine and simazine, MTK showed a removal between 90% (atrazine)
and higher than 99% (simazine). The recoveries employing solutions at initial concentra-
tions of 100lgL�1 of each herbicide showed results of 90% (simazine) and 94% (atra-
zine), whereas for 10 lgL�1, the results of 73% (simazine) and 81% (atrazine) were
obtained. On the other hand, ametryne showed poor recovery values (25 to 40%), probably
due to a stronger interaction with MTK, lowering the recovery values. Based on the results
for atrazine and simazine, MTK presented good features to be used as sorbent phase and for
preconcentration, being easily prepared with low cost, demanding low amounts to be used
for this purpose, providing fast sorption of atrazine and simazine, and with appropriate
The class of triazines comprises one of the main herbicides groups employedaround the world, and has been used as selective pre- and post-emergence herbicidesin crops such as wheat, maize, barley, sorghum, and sugar cane. These herbicidesexhibit moderate interaction with soil components; therefore they have a high
Received 20 July 2012; accepted 18 August 2012.
The authors are grateful to Cooperacao de Aperfeicoamento de Pessoal de Nıvel Superior (CAPES)
for financial support (PROCAD 0082=05-8) and for fellowships.
Address correspondence to Gilberto Abate, Departamento de Quımica, Universidade Federal do
mobility through the soils and because of their widespread use, they are frequentlydetected in surface and ground waters (Katsumata et al. 2006; Jiang et al. 2006;Nevado et al. 2007; Djozan et al. 2012). Herbicides such as Atrazine (AT); Simazine(SIM); propazine (PROP); and the main AT metabolites – deethylatrazine (DEA),deisopropylatrazine (DIA), and didealkylatrazine (DDA), have been considered bythe USEPA as potential compounds for disrupting the endocrine systems ofmammals and humans (Katsumata et al. 2006; Jiang et al. 2006; Nevado et al. 2007).
The most appropriate analytical techniques used to determine triazines herbi-cides and their metabolites in different environmental matrices are gas chromato-graphy (GC) (Jiang et al. 2006; Djozan et al. 2010; Djozan et al. 2012) and highperformance liquid chromatography (HPLC) (Katsumata et al. 2006; Abate andMasini 2005; Moral et al. 2008; Barchanska, Rusek and Szatkowska 2012). The for-mer has the advantage to present a higher sensitivity and selectivity and has beenused for determining volatile and thermally stable pesticides. On the other hand,HPLC has been widely used, especially for more polar pesticides (Moral et al.2008), which cannot be determined by GC.
In spite of the excellence of these analytical techniques to quantify triazinesherbicides, the sample pretreatment involving extraction and preconcentration isof prime importance. The usual liquid-liquid extraction (LLE) (Dean, Wade, andBarnabas 1996; Tran, Hyne and Doble 2007) has been applied to aqueous samples;however, it is a time-consuming method, demanding the use of high solvent volumeswith high cost, and producing high amounts of hazardous waste. As a result, thenumber of methods employing solid phase extraction (SPE) has increased in therecent years for triazines quantification in aqueous media, especially C-18 phase(Jiang et al. 2006; Nevado et al. 2007; Dean et al. 1996; Tran et al. 2007; Barchanskaet al. 2012), owing to characteristics like easy handling, fast and high selectivity(Breton et al. 2006). Notwithstanding this, the common commercial SPE cartridgesare expensive, requiring many times toxic solvents for the elution (Katsumata et al.2006) and are rather nonspecific (Breton et al. 2006).
Because of the drawbacks of LLE and SPE extraction and preconcentrationmethods, a great number of alternative materials or techniques have been investi-gated for triazines analysis. Some of the papers include: stir-bar sorptive extraction(Sanchez-Ortega et al. 2009), molecularly imprinted polymers (Breton et al. 2006;Djozan et al. 2010; Djozan et al. 2012), cloud point extraction (Takagai and Hinze2009), multiwalled carbon nanotubes (Zhou et al. 2006; El-Sheikh et al. 2008;Katsumata et al. 2010), immunosorbents (Vera-Avila et al. 2005), immersed solventmicroextraction (Bagheri and Khalilian 2005), diatomaceous earth (Katsumata et al.2006), and modified alumina (Moral et al. 2008).
As a result of the great availability and low price of the clay minerals, the inter-actions with herbicides have been studied with no previous treatment or chemicallymodified in order to reduce the contamination of soils (Cruz-Gusman et al. 2004;Nasser et al. 2009), for the development of formulations with controlled release insoils (Gerstl, Nasser, and Mingelgrin 1998; Li et al. 2009; Park, Ayoko and Frost2011) and for removal of triazines and other herbicides from waters (Abate andMasini 2005; Azejjel et al. 2009; Park et al. 2011). These applications are feasible,since clay minerals have important characteristics such as high surface area andsorption capacity (McBride 1994). Clay minerals have a high and strong interaction
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process with triazines, even with no surface modifications (Abate and Masini 2005;McBride 1994). It is probable that the pH of the interlamellar region of the clayminerals is lower than the pH of the bulk suspension, providing the protonationof the triazines molecules and consequently their sorption (McBride 1994). Theseclay mineral properties could provide the use for analytical purposes that have beenstudied for preconcentration of metallic ions (Rajesh, Mishra, and Pareek 2008). Onthe other hand, studies focused on the preconcentration of herbicides or otherorganic pollutants on to clay minerals, rarely have been studied. Zarpon et al.(2006) evaluated the use of montmorillonite in order to carry out the preconcentra-tion of the herbicides AT and PROP and the main AT metabolites DEA, DIA, andhydroxyatrazine (ATOH). According to the authors, the recovery percentages forthe compounds, except for ATOH, in aqueous medium was between 85% (DEA)and 98% (PROP), and these results are similar to those obtained with the commercialC-18 cartridges for the same compounds (Zarpon et al. 2006). These findings indicatethat the potassium MT could be an appropriate material for preconcentration oftriazine herbicides with adequate recoveries, low cost, easy availability, and goodchemical resistance. According to de Rezende, Peralta-Zamora and Abate (2011),this clay mineral showed no interaction with the herbicides dichlorophenoxiaceticacid (2,4-D), metolachlor, alachlor, and a minimum sorption with diuron, while asignificant sorption process was observed for AT, AM, and SIM. This featuresuggests that MT could be employed as selective material for extraction and precon-centration of triazine herbicides in aqueous medium.
Several papers have described the interaction between triazines herbicides withclay minerals for environmental applications with emphasis for atrazine; although,from the point of view of analytical applications for triazines, the literature is scarce(Zarpon et al. 2006; de Rezende et al. 2011), and no appropriate results of sorptionor recovery have reported for SIM and AM. Despite of this, these previous resultsfrom the literature suggest a relevant sorption process between triazines and mont-morillonite. Thus, the goal of the present work was to explore the potentiality of acommercial montmorillonite, previously purified and treated with potassium (MTK),sodium (MTNa), and calcium (MTCa) in order to evaluate the preconcentrationprocess of the herbicides AT, SIM, and AM, at concentration levels of mgL�1, usingHPLC for quantification. Furthermore, it was investigated the influence of the inter-lamellar cations (Kþ, Naþ, and Ca2þ) in the sorption process, that was an aspect notpreviously evaluated for triazines herbicides.
MATERIALS AND METHODS
Reagents and Equipment
The analytical standards of the herbicides AT, SIM, and AM, were acquiredfrom Riedel-de Haen, and stock solutions of each were prepared in methanol atconcentration of 500.0 mgmL�1 and stored in a freezer (�18�C). Table 1 showsthe structures of the compounds, as well as some physicochemical properties. Thesolvents methanol and acetonitrile (HPLC grade) were supplied by J.T. Baker.The acid-activated montmorillonite clay (MT-K10) was obtained from Aldrich.The water used in all experiments and in the composition of mobile phase was
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distilled and deionized, using the Millipore Simplicity UV, coupled to an UV lamp.The salts NaCl, KCl, and CaCl2 and other reagents employed in the present workwere of analytical grade and supplied by Merck, Sigma, Carlo Erba, J.T. Baker orsimilar. A Hanna potentiometer (pH 21) coupled to an Ag=AgCl combined glasselectrode was used to measure the pH. The surface area was obtained from BETmeasurements of N2 adsorption using an ASAP 2010 from Micromeritics Instr.Corp. The basal spacing of MTK, MTNa, and MTCa was determined by X-raydiffraction (XRD) employing a XRD-6000 diffractometer from Shimadzu.
HPLC and Chromatographic Conditions
A Shimadzu LC-10AT HPLC coupled to a photodiode array detector(PDA-M10A) was employed, and the acquisition of the signals was made in220 nm using the software LC solution. The sample injection was done with a rotaryRheodyne valve using a 20 mL sample loop. An octadecylsilano (C-18) column,Shim-pack CLC-ODS (5 mm, 4.6mm� 150mm) was used, connected to a C-18guard column, Shim-pack G-ODS (5 mm, 4.0mm� 10mm). The mobile phase pre-viously filtered in 0.45 mm PTFE membrane (Millipore) was constituted by deionizedwater (40%) and acetonitrile (60%) under isocratic mode. The analytical curves wereobtained with five standards, containing AT, SIM and AM at concentrations of0.025, 0.050, 0,500, 1.000, and 2.500 mgmL�1. All the samples and standards werefiltered through disposable 0.45 mm PTFE membrane from Millipore, with 25mmof diameter, before the HPLC analysis.
Montmorillonite Treatment
A mass of 5.00 g of MT-K10 was treated with 50mL of 0.50mol L�1 HClsolution under gentle agitation for 30min in an orbital shaker, and separated by
Table 1. Structures and some characteristics of the herbicides
Characteristic Atrazine Simazine Ametryne
Structure
Molar mass (gmol�1) 215.5 201.5 227.0
Water solubility (mgL�1, 25�C)a 70 5 185
pKaa 1.68 1.65 3.93
logKOWb 2.5 2.1 2.6
Vapour pressure (�10�6 mmHg,
20�C)a0.3 0.0061 0.84
LD50 in rats (mg kg�1)a 3080 5000 1100
Standard Purity (%)c 99.0 99.0 98.5
aData from Dean et al. 1996.bData from Tran et al. 2007.cSupplier information.
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centrifugation, at 2300 rpm for 10min. The supernatant phase was discarded and thesolid phase was treated using 1.0mol L�1 KCl solution for the same contact time,being this step carried out three times. The MTK was washed four times with50mL of deionized water to remove the KCl excess. The same procedure wasrepeated using NaCl and CaCl2, to obtain MTNa and MTCa, respectively. Next,the slurry of MTK, MTNa, and MTCa were dried at 60�C and crushed, being deter-mined the percentage of particles between 100 and 195 mesh. The solids were storedin a desiccator for the sorption and desorption experiments.
Sorption and Desorption Experiments
Solutions of 20mg of MTK, MTNa, or MTCa were transferred to glass centri-fuge tubes and 5.00mL of 2.50 mgmL�1 AT, AM, or SIM solutions were addedindividually, or a mixture of these three herbicides at the same concentration. Thetubes were maintained under mild shaking for 15min at room temperature and cen-trifuged at 2300 rpm for 10min. The supernatant phases were carefully separatedand filtered in 0.45 mm Millipore membrane for HPLC quantification to evaluatethe sorption process. A blank experiment was carried out in parallel.
An additional group of experiments was done employing only MTK. In thissituation, 120mg of MTK was transferred to glass centrifuge tubes, in the presenceof 50.0mL of each herbicide or 50.0mL of the mixture of the herbicides, in aconcentration of 100 mgL�1 or 10 mgL�1 each one. The tubes were agitated and cen-trifuged as previously described, and the liquid phases removed for HPLC analysis.An aliquot of 0.80mL of acetonitrile (ACN) was added to the solid phase, the tubewas shaken manually for one min, centrifuged for 10min, and the organic phase wasreserved. Two additional ACN extractions were made and the three ACN superna-tants were combined. The volume was completed to 5.00mL with deionized water,and the solution was filtered before HPLC analysis, in order to assess the recoverypercentage. This experiment was repeated using a single extraction step with1.50mL of ACN. In both set of experiments, a blank test was carried out.
RESULTS AND DISCUSSION
Sorbent Characteristics
The MT-K10 material was previously treated with 0.50mol L�1 HCl for theremoval of mineral impurities from MT-K10. After that, the Kþ, Naþ, and Ca2þ
ions were employed to evaluate if different interlamellar cations could provide differ-ences in the sorption process of the herbicides, as well as to obtain homoionicmaterials. The MTK, MTNa, and MTCa showed near 80%, 75%, and 86% of the par-ticles between 100 and 195 mesh, respectively. This is not a significant difference andmay be related to the grinding process. The BET surface area showed similar resultsfor MTK, MTNa, and MTCa, between 220 and 242m2 g�1 that is comparable to thesupplier information of 240m2 g�1, and for other potassium saturated MT of228m2 g�1 (Abate and Masini 2005). Conversely, these results are different fromthe literature data that reports values between 600 and 800m2 g�1 (McBride 1994;Sparks 2003). The basal spacing d(001), was 0.99 nm for the three materials whereas
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for MT-K10, it was 1.00 nm, that is, practically there are no differences due to vary-ing ion treatment. These results are not in good accordance with the literature, withcited values of 1.30 nm (Abate and Masini 2005), 1.16 nm (McBride 1994), and1.40 nm (Sparks 2003), for potassium montmorillonites. Moreover, higher d(001)values for MTNa and MTCa in comparison with MTK were expected, since both ionshave a higher hydration degree (McBride 1994). According to Pinto, de Oliveira, andRibeiro (2008), the MT-K10 was prepared from a natural montmorillonite that wascalcined at 873K, followed by acid washing that generated a material with poorercrystallinity. The thermal treatment of this material at 823K caused a diminutionof basal spacing from 1.4 to 1.0 nm, as a result of a structural collapse (Sparks2003). This may be the reason for the same basal spacing for the materials thatallows one to conclude that no change in the interlamellar region occurred. Despitethis, strong acid treatment may produce a material with higher sorption capacity,due to the exposition of surface hydroxyl groups (Rajesh et al. 2008). Someadditional details about the characterization of the Aldrich montmorillonite K-10are discussed in the literature (Abate and Masini 2005; de Rezende et al. 2011; Pintoet al. 2008).
HPLC Quantification
The triazines determination has been frequently carried out by HPLC (Abateand Masini 2005; Katsumata et al. 2006; Zarpon et al. 2006; Moral et al. 2008)and employed water or buffer solution and acetonitrile as the mobile phase. Theseparation of the three compounds was performed under the isocratic mode, withretention times of 3.52min for simazine, 4.81min for atrazine, and 6.66min for ame-tryne. The analytical curves obtained between 0.025 and 2.500 mgmL�1 showedsimilar behavior for ametryne and atrazine with a slightly higher sensitivity incomparison to simazine. All the analytical curves showed r2 values between 0.9993and 0.9999, with a limit of detection (LOD) near 0.007 mgmL�1, and a limit ofquantification (LOQ) close to 0.02 mgmL�1, being the first point (0.025 mgmL�1)adopted as the lower concentration to obtain the analytical curve, since a good repe-titiveness with small standard deviation was observed for this value and for the otherconcentrations.
Sorption Performance of MTK, MTNa, and MTCa
This study was performed at concentrations of 2.50 mgmL�1 with each herbi-cide or the mixture of AT, AM, and SIM in 15min of contact time with the clayminerals. The contact time was adopted based on a previous study, as well as onthe basis of the literature (Zarpon et al. 2006). The objective of this procedure wasto evaluate the sorption process of the clay minerals, being a quick sorption processessential due to the analytical purpose of this study; hence, longer contact times arenot desirable. Figure 1 shows the chromatograms of this study.
It is important to mention, that the blank experiments of the mineral phasesshowed no measurable peaks under the experimental conditions (Figure 1) that isin good agreement with the literature (Zarpon et al. 2006). This indicates that theacid and the ionic treatments were appropriate for this study, at least for the
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HPLC-UV detectability. According to Figure 1, MTCa, MTNa, and MTK showed asuitable performance for removal of AM, AT, and SIM, by comparison of the peakareas with the standard at initial concentration of 2.50 mgmL�1. Moreover, the chro-matogram of MTK indicates the better performance of this material, based on thelow peak area of AT, and the absence of peaks in the region of SIM and AM, sug-gesting a complete removal of these herbicides. The sorption results are shown inTable 2, for the study with each herbicide and for the mixture of the three herbicides.
The better performance as sorbent was verified for MTK that showed the mini-mum retention percentage of 90% for AT. The MTCa demonstrated a moderate per-formance for AT and SIM, while the removal percentage of AM was higher than99%, which was the best result between the herbicides, also verified for MTNa andMTK, as this result was based on the value of 0.025 mgmL�1, adopted as the LOQof this method. The results demonstrated a better behavior of MTK compared toMTNa and MTCa for the sorption of the herbicides that is in good agreement withthe literature for a study between calcium or potassium smectite with atrazine (Chap-pell et al. 2005). The sorption results for MTK in this study was higher than reportedin a previous study (de Rezende et al. 2011) using similar conditions, where obtainedresults were 87%, 69%, and 56% for AM, SIM, and AT, respectively; however, in this
Table 2. Sorption (%)a of AT, AM, and SIM onto MTK, MTNa, and MTCa
Individual study Mixture of the herbicides
Clay mineral AT AM SIM AT AM SIM
MTK 95� 1 >99 >99 90� 1 >99 98� 1
MTNa 54.8� 0.7 >99 94� 2 48� 3 >99 93� 1
MTCa 50� 4 >99 90� 4 33� 1 >99 79� 4
Note: Initial concentration of the herbicides: 2.50mgmL�1.aResults express the average of three experiments.
Figure 1. Chromatogram of a standard solution of SIM, AT, and AM at concentration of 2.50mgmL�1;
chromatograms of the same standard solution, but after the contact time with MTNa, MTK, and MTCa for
15min; and the blank chromatogram.
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article the authors suggest that the blank signals could compromise the results, as thequantification method was spectrophotometric and not based on separation bychromatography. The comparison of the results for herbicides in mixture and forthe individual study, showed practically no differences, except for MTCa with ATand SIM. This indicates a lower availability of sorption sites for MTCa, as a reason-able reduction in the sorption percentage was observed, although for AM a completesorption was verified. Although the sorption isotherms were not carried out, it isimportant to report that the sorption percentage for the three herbicides togetherwas higher than 99% for AM (Table 2) suggesting no sites saturation. This is sup-ported by the literature (Zarpon et al. 2006) that reports at least 95% of AT removalby potassium MT in 15min of contact time, with a 10.0 mgmL�1 AT solution, incomparable conditions of the present work and considering the concentration of2.50 mgmL�1 for each herbicide. Moreover, the principal focus of the present studywas to verify the behavior of the clay minerals under conditions of low herbicidesconcentrations, and, in addition, to emphasize the previous literature’s results(Zarpon et al. 2006; de Rezende et al. 2011) for triazines herbicides, especially withregard to the study of the sorption and recovery process. These previous results andthe behavior of the MTK in the present work, suggested that all triazine herbicidesare easily sorbed by this mineral phase. This is a feature not reported by the litera-ture, at least until the present moment.
If the interaction between the herbicides and the mineral phases occurs byinterlamellar change, it is probable that the MTCa should have a higher percentageremoval. The smaller hydration degree of Kþ ions could cause a drawback for itsremoval from the interlamellar region of the clay minerals, because these ions wouldbe directly associated with the mineral surfaces of the clay, by cation bridging(Sparks 2003; Sposito 2008). On the other hand, the water molecules from the morehydratable calcium or sodium ions would be bound to the mineral surfaces by waterbridging that could make possible the removal of this cation for the change with theprotonated triazines (McBride 1994; Sparks 2003; Sposito 2008). Notwithstanding,the pH of the suspension was near 6.5, that is, not favorable to protonate the herbi-cides, although the pH in the interlamellar region could be lower than the bulksuspension (McBride 1994; Site 2001). In fact, the complete removal of AM fromthe solution, suggest that the process was based on the presence of the protonatedAM, due to its higher pKa value of 3.93 (Table 1), providing an easier protonationprocess of the NH groups that could aid the electrostatic attraction by the negativelysurfaces of the clay mineral (McBride 1994). Thus, it is expected that for AT andSIM, the molecules would be sorbed by van der Waals forces and hydrogen bonding,due to their lower pKa values, whereas for AM sorption, an additional electrostaticmechanism could occur. Indeed, the pH of the bulk suspension seems to be animportant factor for triazines sorption onto clay minerals (Site 2001). In a sorptionstudy between sodium MT with AT and SIM, Polati et al. (2006) verified moderatesorption values between 35 and 40% for both herbicides, but for a pH value of 9.9,that is, a condition highly favorable to a sorption as neutral molecules (Site 2001).Lowering the pH from 7.2 to 2.5 resulted in a significant enhancement of the ATsorption onto vermiculite near to 7% to 80%, suggesting the sorption of the proto-nated specie (Abate and Masini 2005). The results of the present study for MTK
are in good accordance with the literature (Abate and Masini 2005) that showed
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AT removal percentage higher than 99.5% for potassium MT. Because of the bettersorption performance for MTK, this sorbent phase was selected for an additionalsorption process, as well as a recovery study, but using lower concentrations ofthe herbicides.
Sorption and Recovery Study
It is important to notice that two different concentrations were used in thisstudy, providing a preconcentration factor of ten times. Thus, the concentrationof 0.010 mgmL�1 was used as the lower concentration, to provide near0.100 mgmL�1 in the final herbicides solution, hence, approximately four timeshigher than the LOQ of the HPLC in the present study. The analysis of supernatantphases from the study of 0.010 mgmL�1 or 0.100 mgmL�1 presented no measurablechromatographic peaks, being the baseline of the chromatograms analogous toblank analysis that was predictable, since a high sorption degree was verified inthe first part of this study. This hindered a direct evaluation of the sorption process,although the absence of chromatographic peaks for AM, AT, and SIM for the initialconcentration of 0.100 mgmL�1 indicate a removal close to 90%, based on the LOD(0.007 mgmL�1). Table 3 shows the results obtained for the recovery study, usingMTK as sorbent phase owing to the better sorption behavior of this sorbent.
Higher recovery values were observed to three extractions in comparison with aunique extraction step, except one value for AM, suggesting the importance of carryout three or at least two extractions, for an improvement in the recoveries. Appro-priate results were observed for AT and SIM in both initial concentrations, althoughsuperior results were clearly obtained for the higher initial concentration. This couldbe ascribed to a partially irreversible sorption process in the time scale of the experi-ment, for the smaller concentrations, considering the use of acetonitrile. Neverthe-less, the recovery results employing this solvent was close to 100% for atrazineand their degradation products (Abate and Masini 2005), for a contact time of30min using two cycles of extraction, against three cycles of 1min in the presentwork. Perhaps this is the cause for the small recovery verified for AM in both con-centrations that is reinforced by the high sorption percentages, according Table 2,suggesting a more intense interaction process between AM and MTK. Despite thelow recovery of AM, the process provided recoveries values of 73% and 81%for SIM and AT for the initial concentration of 0.010 mgmL�1, while for
Table 3. Recovery (%)a of AT, AM, and SIM from MTK
0.100mgmL�1 0.010mgmL�1
AT AM SIM AT AM SIM
1 extraction 84� 3 34� 6 92� 2 77� 2 27� 2 66� 6
3 extractions 94� 6 25� 2 90� 7 81� 5 40� 1 73� 2
Note: Initial concentration of each herbicide: 0.100mgmL�1 or 0.010mgmL�1, in a
mixed solution.aResults express the average of three experiments.
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0.100 mgmL�1, between 90 and 94% were observed for the same herbicides, forthree extractions steps, that are acceptable values. Also, on the basis of the smallerpolarity of AM, since the higher retention time was 6.66min, it would be possiblethat the use of a less polar solvent could contribute to a more efficient AM extractionfrom the MTK. In addition, AM may be more liable for a protonation process,according its pKa value, as previously discussed. As reported in the literature,recoveries results between 90% and 99% were obtained for atrazine and propazineherbicides, but using a solution containing KCl and ACN as extractant. Accordingto the authors, this solution presented better recoveries values in comparison withthe use of pure ACN (Zarpon et al. 2006). Although in the present study, nonatural water samples were employed, it is important to notice that this was inves-tigated in a previous paper (Zarpon et al. 2006), including a study of the influenceof humic acid.
Features of the Method and Comparison with the Literature
In general, the performance of MTK as sorbent phase showed remarkable char-acteristics, and comparable with other different approaches using solid phases pre-sented in the literature for preconcentration and determination of triazines. Table 4
Table 4. Comparison of the proposed method with other methods using alternative approaches for
preconcentration and determination of triazines herbicides in water samples
Sorbent Analyte
Range
(mgL�1)
LOD
(mgL�1)
RSD
(%)
Recovery
(%) Reference
Diatomaceous eartha AT 1–150 0.24 4.7 95.7 Katsumata et al. (2006)
SIM 1–300 0.21 2.7 75.0
Multiwalled carbon
nanotubesaAT 0.2–100 0.033 0.6 60–105d Zhou et al. (2006)
SIM 0.02–100 0.009 2.1 70–105d
Potassium
MontmorilloniteaSeveral 0.4– 5.0 0.05– 1.0– 63–99 Zarpon et al. (2006)
0.15 7.4
Surfactant-coated
aluminaaAT – 0.004 – 93–107 Moral et al. (2008)
SIM – 0.004 – 97–103
DEA – 0.016 – 31–43
Multiwalled carbon
nanotubesaAT 10–100 0.058 0.2– 81–108 El-Sheikh et al. (2008)
2.8
Multiwalled carbon
nanotubesbAT 0.1–1.0 0.025 6.9 87–110 Katsumata et al. (2010)
summarizes the principal figures of merit of some methods reported in the literaturefor different triazines. Conventional solid materials (e.g., C18 phase) are not pre-sented in this table.
Considering the results from three extractions, shown in Table 3, AM showed noadequate recovery, as earlier described, due to the strong interaction with the mineralphase. On the other hand, for AT and SIM, the recoveries values shown are in goodagreement with other approaches from the literature, as well as the RSD. It is impor-tant that the range and LOD of the present method are relatively higher than the othermethods when HPLCwas employed. This is due to the different ways to determine thevalues, the most values presented in Table 4, were obtained by using three times theratio signal=noise. Usually, the factor of preconcentration is considered to estimatethe LOD and the analytical range, which is contrary to this work. The LOD was cal-culated using three times the standard deviation of the blank signal (s), divided by theslope (S) of the analytical curve (3 s=S), whereas the LOQ was determined using 10 s=S. If the preconcentration factor of ten times is considered, the LOD and LOQ valuesof 0.7mgL�1 and 2.0mgL�1 would be achieved, respectively. Even so, the LOD valueis higher in comparison with other HPLC methods (Table 4), which is related to thepreconcentration factor, sometimes between 50 and 200 times. In the present methodthe batch approach was used, and higher volumes of sample could be employed. How-ever, a technical drawback was the availability of an appropriate centrifuge in order toemploy higher volumes. Notwithstanding this, the principal purpose of this work wasto investigate the performance of MT in the sorption of different triazines, besides toevaluate the influence of the interlamellar cation. In addition, it is important to notice,that the results here presented, corroborate the hypothesis of a suitable selectivity ofMTK by triazines herbicides, as proposed by the literature (de Rezende et al. 2011).As a result, this sorbent can be considered an adequate material for preconcentrationof triazines.
CONCLUSION
The treatment of MT-K10, with potassium, calcium, and sodium apparentlydid not provide a saturation of the interlamellar regions in the clay minerals, basedon the same d(001) values. In spite of this, the sorption performance of the three dif-ferent materials was very promising, demonstrating a great potential for preconcen-tration of the herbicides AT, SIM, and AM. The study showed a better sorptionprocess for MTK in comparison to MTNa and MTCa, for the three herbicides, pre-senting the lower value of 90% for AT sorption, and higher than 99% for AM.The recovery study employing three extraction steps showed adequate results forAT and SIM, while low recoveries were verified for AM, due to a stronger interac-tion of AM with MTK. This clay mineral is a low cost material, demanding a simpleprevious treatment to produce the MTK, suitable for preconcentration of AT andSIM, for concentrations near 7.5 mgmL�1, with 15min of contact time, using only120mg of the sorbent for each analysis. The results of the present work, allied toa previous paper (Zarpon et al. 2006) related to the sorption and recovery of otherstriazines, suggest the use of MTK as a promising sorbent phase for this importantclass of herbicides.
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