Journal of Colloid and Interface Science 299 (2006) 88–94www.elsevier.com/locate/jcis
Adsorption of atrazine on soils: Model study
Ilias D. Kovaios a,b,∗, Christakis A. Paraskeva a,b, Petros G. Koutsoukos a,b,Alkiviades Ch. Payatakes a,b
a Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation for Research and Technology, Hellas, P.O. Box 1414,GR-26500, Patras, Greece
b Department of Chemical Engineering, University of Patras, GR-26500, Patras, Greece
Received 22 November 2005; accepted 30 January 2006
Keywords: Atrazine; Adsorption from solution; Thermodynamics of adsorption; Adsorption isotherms; Silica; Alumina; Calcium carbonate
1. Introduction
Atrazine [2-chloro-4-(ethylamino)-6-isopropylamino-s-tria-zine], a selective herbicide of the s-triazine chemical family,has been used extensively since the 1950s as a pre- or postemer-gence controller of annual grasses and broadleaf weeds, mainlyin corn and sorghum cultures [1]. The mechanism of action ofatrazine is inhibition of photosynthesis by blocking the elec-tron transfer in photosystem II [2]. Atrazine is among the mostwidely applied herbicides in a number of countries and be-cause of its widespread use, relatively high chemical and bi-ological stability in soils and aquifers [3–6], and high leachingpotential, has in several instances been reported to be present
in surface and ground waters at alarmingly high concentrationlevels [7–10]. EU Directive 98/83/EC, on the quality of waterintended for human consumption, has set the maximum con-centration of atrazine to 0.1 µg/L and the total concentrationof all pesticides to 0.5 µg/L. The effects of atrazine on humanhealth, although controversial to some extent, have been shownto induce increased risk of cancer in humans associated withatrazine exposure [11–14]. Atrazine has in general a negativeimpact on aquatic ecosystems [15], and it has been reported toinduce severe hormonal disturbances in amphibians [16] andtumors in rats [17].
One of the major environmental processes that determine thefate of a chemical substance in soil and its potential to reachgroundwater is sorption on organic and inorganic soil con-stituents [18,19]. Aluminosilicate clay minerals are very impor-tant and chemically active inorganic soil components believedto play a key role in soil processes. Another important factor
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is the organic matter present in soils [20], consisting mainlyof humic substances [21,22]. It is widely accepted that organicmatter plays a major role in sorption phenomena of organicchemicals in soils, mainly for nonpolar substances [23–27].However, it has also been shown that the contribution of theinorganic soil components, and in particular the clay fractionof soil, to sorption processes is significant [28–31], mainly inanhydrous environments, because of the competition betweenwater and organic molecules for adsorption sites on mineralsurfaces [32,33]. The adsorption of atrazine onto inorganic soilcomponents has been studied by several investigators [29–31,34–36].
Despite the fact that a considerable amount of work has beendone on atrazine sorption on clay minerals, studies focused onatrazine interactions with the main constituents of these miner-als (alumina and silica) are still lacking. The present work aimsat contributing to a better understanding of atrazine interac-tions with the main inorganic constituents of clay minerals andchalky soils (silica, alumina, and calcite). The issue of uptakeof atrazine and its reversibility under different solution condi-tions of ionic strength, pH, and temperature was addressed first.All materials investigated developed electric surface chargecaused by interactions of the surface groups and/or potentialdetermining ions with the electrolyte solutions. For this rea-son, atrazine uptake was investigated at general different valuesof ionic strength and solution pH. The surface charge of theadsorbents was measured to estimate the significance of thisparameter for the adsorption process. Measurements of uptakeover a relatively wide temperature range allowed estimationof the thermodynamic parameters of the uptake of the adsor-bate.
2. Experimental
2.1. Materials
Silica gel (silica gel 60, particle diameter 40–63 µm, Merck),γ -alumina (aluminum oxide anhydrous, Merck), and calcite(CaCO3, Merck) were used as model inorganic solid substrates.Atrazine (99.2% Pestanal analytical standard, Sigma–Aldrich)was chosen as the prototype herbicide. Selected properties ofthese substances are presented in Tables 1 and 2. Alachlor(99.9% Pestanal analytical standard, Sigma–Aldrich) was usedas an internal standard for GC analysis. Dichloromethane(99.9% for GC, Fluka) and ethyl acetate (99.5% for GC, Fluka)were used for extraction of atrazine from water samples andas a solvent for GC analysis, respectively. Conductivity wa-ter was used throughout the experiments (Milli-Q Plus, Milli-pore). Atrazine was analyzed by UV–visible spectrophotometry(UV-1601, Shimadzu) or by gas chromatography with FTDdetector (GC 17-A, Shimadzu). The specific surface area ofadsorbents was measured by nitrogen adsorption using theBET method (Micromeritics, Gemini). Particle size distribu-tions of the adsorbent powders were measured using a laserlight-scattering technique (Mastersizer S, Malvern InstrumentsLtd). Zeta (ζ ) potentials of silica were measured by the stream-ing potential technique.
Table 1Selected physicochemical properties of atrazine
Chemical formula C8H14ClN5
Chemical structure
Molecular mass 215.7 g mol−1
Water solubility at 20 ◦C 28 mg L−1
Density at 20 ◦C 1.187 g ml−1
Melting point 173–177 ◦CVapor pressure 3.85 × 10−5 PapKa 1.7
The time needed for the attainment of equilibrium betweenthe liquid and the solid substrates was measured by batch exper-iments. Precisely weighted amounts of absorbent were placedin glass vials with Teflon-line screw caps containing only thebackground electrolyte (sodium chloride) to ensure constantionic strength. The vials were placed for 24 h in a constant-temperature chamber with continuous end-over-end mixing.After this time interval, sufficient for the solid substrates toreach equilibrium at the experimental ionic strength, equal vol-umes of stock atrazine solution of the same ionic strength valuewere added to the vials. The presence of the same initial con-centrations of atrazine and a constant liquid-to-solid mass ratiowere thus achieved. The time interval of atrazine solution addi-tion was recorded and the vials were placed again in the cham-ber for mixing at constant temperature. The vials were openedat random time intervals, and the suspensions were centrifugedand filtered by 0.22-µm PTFE membranes (Millipore). Thesupernatant solutions were diluted if needed and the atrazinein the aqueous medium was extracted with dichloromethane(DCM). The solvent was exchanged with ethyl acetate, somequantity of internal standard was added, and the samples wereanalyzed by gas chromatography with FTD detector. All exper-iments were done at least in duplicate.
2.3. Adsorption isotherms
The adsorption isotherms were determined by batch exper-iments. The procedure of equilibration of the adsorbents withthe electrolyte solutions was the same as described above forthe kinetics measurements. Past the equilibration period, dif-ferent volumes of stock atrazine solutions were added to vials,resulting in different initial concentrations of atrazine and con-stant liquid-to-solid mass ratio at constant ionic strength. The
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solid-to-solution ratios used for the various substrates were10 ml/g for adsorption on silica gel and 2 ml/g for adsorp-tion on γ -alumina and calcite. Next, the vials were placed inthe thermostated chamber for an additional 48-h equilibrationperiod to attain equilibrium. After this, the suspensions werecentrifuged and filtered from 0.22-µm PTFE membranes (Mil-lipore). The supernatant solutions were diluted as needed andanalyzed by UV spectrometry at 220 nm. The pH adjustmentfor adsorption experiments at different pH values was achievedby addition of small aliquots of concentrated stock sodium hy-droxide or hydrochloric acid as needed. It should be noted thatthe additions needed did not affect the ionic strength of the solu-tions to any significant extent. All experiments were done threetimes. The surface excess concentration of the adsorbate (Γ )was calculated using the equation
(1)Γ = (C0 − Ceq)V
wSA,
where C0 and Ceq (mg/L) are the initial and the residual con-centrations of the adsorbate in the suspension, V (L) is thevolume of the suspension, and w (g) and SA (m2/g) are themass and the specific surface area of the adsorbent, respectively.
2.4. Streaming potential measurements
The streaming potential and the zeta potential measurementsof the silica gel suspensions were measured at different pHvalues using a fully computerized apparatus. Details of the ap-paratus and the experimental technique used have been reportedelsewhere [37].
3. Results and discussion
3.1. Characterization of substrates
Particle size distribution, mean diameter values, zeta poten-tials, and specific surface areas (SSA) of the adsorbents usedin the present work are summarized in Table 2. The high spe-cific surface area of silica is one of the reasons for which ithas attracted considerable attention as a substrate for adsorption[38–40]. From the shape of the nitrogen adsorption isothermsobtained and from porosity measurements, it was concludedthat the large SSA of the silica substrate used in the presentwork was due to intraparticle spacing.
3.2. Adsorption isotherms
Adsorption from solution may be defined as the localizationof solute on the surface of a solid, while absorption is charac-terized by the formation of a solution of the solute in a solid.These two processes cannot easily be differentiated if the solidis porous and has a large internal surface [41].
Atrazine was not adsorbed (within the limits of the analyti-cal method used) on the surface of γ -alumina and calcite, evenat a low liquid-per-solid ratio of 2 ml/g, for a pH range be-tween 7.7 and 11.7 for calcite and 3.8 to 12.1 for γ -alumina.These results are in agreement with Clausen et al. [35], who
Fig. 1. Kinetics of atrazine adsorption on silica gel. Plot of the surface concen-tration as a function of time. Initial atrazine concentration 5 mg/L, pH 6.1, at25 ◦C, 0.2 M NaCl.
investigated the adsorption of some pesticides on quartz, cal-cite, kaolinite, and α-alumina. They also found no detectableadsorption of atrazine on calcite and α-alumina. In addition,Fruhstorfer et al. [34], studied the adsorption of atrazine onmontmorillonite, a 2:1 clay mineral (an aluminum oxide sheetbetween two silicon tetrahedral sheets) and on kaolinite, a 1:1clay mineral (an aluminum oxide sheet and a silicon tetrahedralsheet alternately), and they found that the montmorillonitic clayadsorbed large quantities of atrazine, contrary to the kaoliniticsubstrate.
Significant adsorption of atrazine was found with the silicagel substrate. The rate of the process is too high to measure ac-curately because adsorbed atrazine on silica gel reaches 75% ofits final amount almost instantly. A plateau value was attainedin about 2 h as may be seen in Fig. 1.
The experimental data were fitted with the Freundlichisotherm [41],
(2)Γ = KFCneq,
where Γ (µg/m2) is the surface concentration of atrazine,KF (µg/m2)(µg/ml)−n is the Freundlich coefficient, whichshows the affinity of sorbate with sorbent, Ceq (mg/L) is theequilibrium concentration of atrazine in electrolyte solution,and n (dimensionless) is the Freundlich exponent, which showsthe energy distribution of adsorption sites.
From the linearized form of the Freundlich model,
(3)logΓ = logKF + n logCeq,
the Freundlich constants KF and n can be calculated.
3.3. Effect of ionic strength (IS)
Adsorption isotherms of atrazine onto silica gel obtained at25 ◦C and for ionic strength values ranging from 0.1 to 0.3 MNaCl are shown in Fig. 2. As may be seen, the atrazine sur-face concentrations increased with increasing ionic strength of
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Fig. 2. Adsorption isotherms of atrazine onto silica gel surface for three differ-ent values of ionic strength at 25 ◦C, pH 6.1: (1) 0.1 M NaCl; (!) 0.2 M NaCl;(P) 0.3 M NaCl.
Table 3Freundlich coefficients KF and exponents n calculated for different values ofsolution ionic strength, pH, and temperature
Temperature (◦C)25 0.0189 0.84230 0.0173 0.87035 0.0166 0.85940 0.0144 0.915
the solution, without in all cases reaching a plateau value. Mea-surements at higher concentrations however were limited by thesolubility of atrazine in water. The solid line drawn through theexperimental points is the fit according to Eq. (2). Using thelinearized Freundlich isotherm (Eq. (3)), the values for the ad-sorption coefficients were calculated and the results are summa-rized in Table 3. A feature shown in Fig. 2 is that the adsorptionof atrazine onto the silica gel particles increases with increas-ing ionic strength. This suggests that the role of electrostaticinteractions in the adsorption process is significant. The val-ues of the exponent n in Eq. (2) (Table 3) were calculated tobe in the range from 0.842 to 0.925. The dependence on ionicstrength has been reported on several occasions [42,43]. Theincreased adsorption suggested that there is no competition ofthe chloride ions for the silica substrate, and the effect of thesalt concentration was to suppress the thickness of the electri-cal double layer at the silica/electrolyte interface. Moreover, thepossibility of surface adsorption of sodium ions would lead tomore positive surface potentials, which are expected to reducethe surface concentration of the negatively charged groups of
Fig. 3. Adsorption isotherms of atrazine onto silica gel surface at different pHvalues at 25 ◦C, 0.2 M NaCl: (1) pH 3.3; (!) pH 6.1; (P) pH 9.1.
the silica substrate. The adsorption of atrazine is most likely lo-cated at the IHP of the electrical double layer. Our results arein good agreement with the report of Xing et al. [44], who in-vestigated the adsorption of atrazine onto a silica gel at lowerionic strength values and found lower values for KF, while forn a value of 0.840 was calculated.
3.4. Effect of pH
The investigation of the pH dependence of the adsorption ofatrazine onto silica gel gave results that corroborate the sugges-tion that surface charge of the adsorbent is an important factor.The pH dependence is illustrated in Fig. 3. A decrease of the so-lution pH from the equilibrium pH value of 6.1 to 3.3 resulted tovery little increase of the adsorbed atrazine. For higher pH val-ues the difference from atrazine adsorbed at the equilibrium pHwas significant. These observations are confirmed by the KFvalues (Table 3). For pH 3.3 KF values are 1.26 times higherthan those corresponding to pH 6.1. At pH 6.1 KF was foundto be 9.45 times higher than the corresponding value at pH 9.1.The amount of adsorbed atrazine on silica gel surface for differ-ent pH values is shown in Fig. 4. Increase of the electrolyte pHis expected to increase the negatively charged surface groups ofthe adsorbent significantly. In this case competition for the neg-atively charged sites by the sodium cations cannot be ruled out.As a consequence, fewer sites are available for the atrazine ad-sorbate, which at the higher pH values is fully deprotonated andmay bind only to the nondissociated ≡Si–OH group, the surfaceconcentration of which is reduced in alkaline pH values.
3.5. Surface charge
Assuming that the atrazine molecule projection onto a planecorresponds to a surface of 61.5 Å2, the maximum surface ca-pacity for atrazine sorption is estimated to be approximately580 µg/m2. This value is significantly larger than the experi-mental one. The relatively small coverage of silica surface byatrazine can be attributed partly to the low concentration of
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Fig. 4. Surface concentration of atrazine on silica gel for different pH values at25 ◦C, 0.2 M NaCl. Initial concentration of atrazine 7.5 mg/L.
active silanol sites (≡Si–OH, where ≡ stands for the solid sur-face) on silica surface, and partly to the extensive pore networkof silica. As it can be seen in Fig. 5, the zeta potential of sil-ica was changes from 0 to nearly −9 mV for pH values from2.9 to 9.5. These zeta potential values are small for silica andindicate that there are few reactive silanol groups on the sur-face. It should be noted, however, that the substrate used in thepresent work is a high-porosity silica with pore volume 0.7–0.8 ml/g and mean pore diameter 60 Å. Surface analysis ofthis silica with Hg porosimetry showed that 78% of the surfacearea is attributed to the pore network. Thus, if an atrazine mole-cule is located at an appropriate position inside a pore, it mayhinder entrance of other molecules because of stereochemicalhindrance, resulting in a considerable reduction of the “effec-tive” surface area of the silica.
Fig. 5 shows the variation of zeta potential on silica gel sur-face for different pH values at constant ionic strength (0.01 MKNO3). Preliminary measurements of electrophoretic mobil-ity and potentiometric titrations have shown that there is nospecific adsorption of K+ or NO+
3 ions on the silica surface.The i.e.p. of this silica is at pH 2.9. Beginning from the i.e.p.at pH 2.9 where the net surface charge is zero, a shift of pHtoward the basic region resulted in the development of a netnegative charge on silica surface, creating more deprotonated≡O− groups. In this pH range, atrazine species exist almostexclusively as uncharged molecules (pKa = 1.7). As can beseen in Figs. 3 and 4, the surface concentration of atrazine onthe silica decreases as pH increases. It may therefore be sug-gested that atrazine is adsorbed mainly as a neutral species onthe uncharged silanol groups (≡Si–OH) of silica. This may beeffected with hydrogen bonds. Atrazine is a weak base witharomatic ring and chain nitrogen atoms which can form hy-drogen bonds with the hydroxyl groups of silica. Davies andJabeen [45] found in a series of IR studies that atrazine isadsorbed onto bentonite and montmorillonite by protonationand/or forming hydrogen bonds. The relatively small electrosta-tic effect measured may be attributed to the relatively low extentof atrazine protonation. Xing et al. [44] also suggests that the
Fig. 5. Zeta potential of silica gel suspensions as a function of pH: 25 ◦C,0.01 M KNO3.
azo and amino nitrogen atoms of atrazine can act as hydrogenbond acceptors with silanol groups or strongly adsorbed wa-ter molecules. Studies of the adsorption of Alachlor onto humicsubstances have also stressed the importance of hydrogen bond-ing in adsorption of pesticides in soils [27]. As can be seen inFig. 4, the surface concentration of atrazine decreases weakly asthe pH of the solution increases from 3 to 6. This trend is morepronounced for pH increase from 6 to 9. These observations arein accord with the proposed mechanism of sorption through hy-drogen bonding, because it is known that silanol groups of silicaare rather insensitive to the development of negative charge overseveral pH units above the pzc. Beyond this pH value chargingis rather easily effected [46]. Thus, adsorption decreases morestrongly at pH values higher than 6 because of the electrostaticrepulsions developed between the silica gel ≡O− groups andthe lone pair electrons of atrazine’s electronegative atoms.
3.6. Effect of temperature
The effect of temperature on the adsorption of atrazine wassignificant and the respective isotherms are shown in Fig. 6.Increase of the system temperature caused a decrease in ad-sorbed atrazine concentration at the same Ceq, suggesting thatadsorption is exothermic. The KF values decreased from 0.0189to 0.0144 (µg/m2)(µg/ml)−n when temperature increased from25 to 40 ◦C. The values of the Freundlich exponents n for thesame temperature range increased from 0.842 to 0.915, giv-ing straighter isotherms (Table 3). The exothermic adsorptionof atrazine on silica may be explained by the fact that theadsorbate–adsorbent bonds weakened with increase of temper-ature [47].
The heat of adsorption for dilute solutions may be obtainedfrom the Clausius–Clapeyron equation,
(4)�Hi = R
[∂ ln(Ceq)
∂(T −1)
]s,
where �Hi is the isosteric enthalpy of adsorption (at constantsolute uptake S) (kJ/mol), R is the universal constant of ideal
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Fig. 6. Adsorption isotherms of atrazine on silica gel surface at different tem-peratures at 0.2 M, pH 6.1: (1) 25 ◦C; (!) 30 ◦C; (P) 35 ◦C; (e) 40 ◦C.
gases (8.3145 J/(mol K)), Ceq is the equilibrium concentrationof solute (mg/L), and T is the temperature (K).
Integration of Eq. (4) results in the linearized form
(5)ln(Ceq) = 1
RT�Hi + C,
where C is the integration constant.The values of �Hi are obtained by plotting the ln(Ceq) ver-
sus 1/T .It should be noted that adsorption reversibility is a prereq-
uisite for the application of Eqs. (4) and (5). Measurementsin batch experiments in which silica particles with adsorbedatrazine were suspended in electrolyte solutions at the sameconcentration, as in the adsorption experiments, showed thatwithin 48 h 75% of the adsorbed atrazine was desorbed. Pro-vided that the desorption process is slow, it was clear that theadsorption could be considered as reversible.
The standard Gibbs free energy change �G0 at differenttemperatures can be calculated from
(6)�G0 = −RT lnK,
where K is the equilibrium constant of the adsorption process.According to Liu [48], K can be reasonably approximated
by bMatr,
(7)K ≈ bMatr,
where b is the affinity constant of the Langmuir isotherm andMatr is the molecular weight of atrazine [48].
The experimental results of adsorption experiments at differ-ent temperatures were fitted to Langmuir model and the valuesof K were estimated. It should be noted that there is a slightdifference between Langmuir and Freundlich model fittings.
Finally, standard entropy changes at different temperaturesand atrazine uptakes are calculated by
(8)�G0 = �H0 − T �S0,
using the values of �G0 and �Hi obtained from Eqs. (5)and (6). (�Hi can be used instead of �H0 as a good approxi-mation.)
Table 4The isosteric heat of adsorption �Hi , the standard entropy change �S0, andthe standard Gibbs free energy change �G0 for atrazine sorption on silica gelat different temperatures and uptakes
Fig. 7. The contribution of isosteric heat of adsorption and entropy change toGibbs free energy change for adsorption of atrazine onto silica gel surface fordifferent surface concentrations of atrazine (1) �H ; (!) −T �S for 298 K <
T < 313 K; (P) �G for 298 K < T < 313 K.
The values of �G0,�Hi, and �S0 are summarized in Ta-ble 4. The isosteric heat of adsorption of atrazine onto silicagel surface was found to be between −17.2 and −6.5 kJ/molfor solute uptake between 0.01 and 0.13 µg/m2. These enthalpyvalues are of an order of magnitude corresponding to physisorp-tion (∼10 kJ/mol) and in the range of hydrogen bonding en-ergetics (10–20 kJ/mol). Similar values have been reportedfor the adsorption of 2,4-dichlorophenoxyacetic acid (2,4-D)onto organophilic sepiolite [49]. The standard entropy rangewas between 18.17 and 53.14 J mol−1 (mean values for differ-ent atrazine uptakes) for a temperature range between 298 and313 K. The standard entropy increase as the atrazine uptake in-creases. This denotes that the order of the system decreases.
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Finally, the standard Gibbs free energy is between −22.97 and−22.10 kJ mol−1 for temperatures between 298 and 313 K,respectively. The negative values indicate that the process ofadsorption is spontaneous. There is not a significant change of�G0 between the temperatures studied. The trends of �Hi,�S0, and �G0 at different temperatures and atrazine uptakesare shown in Fig. 7.
4. Summary
Batch experiments of atrazine adsorption onto model in-organic soil components were performed under different so-lution conditions. Atrazine was not adsorbed appreciably onγ -alumina and calcite over a wide range of pH values. In con-trast, atrazine was adsorbed onto silica gel. Desorption experi-ments of atrazine from silica gel surface showed that the processis reversible. The Freundlich isotherm gives a good fit to thedata. The parameters of the respective isotherms were calcu-lated from the linear plots. Adsorption increased with increas-ing ionic strength of the aqueous medium, indicating significantcontribution of electrostatic interactions. The dependence of ad-sorption on the solution pH, however, indicates that atrazineadsorbs mainly as uncharged species on uncharged sites onthe silica gel surface. Because atrazine is a weak base withfree electron pairs on the N ring and alkyl atoms, it may besuggested that hydrogen bonds are formed between ≡Si–OH(silanol) groups and the nitrogen atoms of atrazine molecules.Finally, the isosteric heat of adsorption, the standard entropychange, and the standard Gibbs free energy change calculatedfrom adsorption experiments at different temperature valueswith the isosteric heat of adsorption were in the range of ph-ysisorption for different atrazine uptakes.
Acknowledgment
The financial contribution of the Institute of Chemical En-gineering and High Temperature Chemical Processes (FORTH/ICE-HT) is acknowledged.
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