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Effects of ionic strength and temperature on adsorption of atrazine by a heat treated kerolite
Chemosphere 59 (2005) 69–74 www.elsevier.com/locate/chemosphere
Effects of ionic strength and temperature on adsorption of atrazine by a heat treated kerolite M.D. Uren˜a-Amate *, M. Socı´as-Viciana, E. Gonza´lez-Pradas, M. Saifi Department of Inorganic Chemistry, University of Almerı´a, La Can˜ada San Urbano s/n, 04120, Almerı´a, Spain Received 19 January 2004; received in revised form 15 September 2004; accepted 29 September 2004
Abstract The adsorption of 6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (atrazine) on a heat treated kerolite sample at 600 C (K-600) from pure water solution at 10 C, 25 C and 40 C has been studied. The influence of the presence of 0.1 M KCl in the medium was also investigated for a better understanding of variables affecting the adsorption of this herbicide. The experimental adsorption data points were fitted to the Langmuir equation in order to calculate the adsorption capacities (Xm) of the samples; Xm values range from 2.3 · 103 mg kg1 (pure water solution at 40 C) up to 15.2 · 103 mg kg1 (0.1 M KCl solution at 10 C). The adsorption data were also fitted to the Freundlich equation in order to clarify the influence of the presence of 0.1 M KCl on atrazine adsorption. The parameter K10 obtained from this equation (adsorption capacity at an equilibrium solution concentration of atrazine equal to 10 mg l1) shows clearly that the presence of 0.1 M KCl in the medium tends to increase the adsorption of atrazine in the range of temperature studied. The adsorption experiment also showed that the lower temperature, the more effective the adsorption of atrazine from both, pure water and 0.1 M KCl solutions. The values of the removal efficiency (R) obtained ranged from 39% at 40 C (pure water solution) up to 93% at 10 C (0.1 M KCl solution). 2004 Elsevier Ltd. All rights reserved. Keywords: Kerolite; Adsorption; Atrazine
1. Introduction Pollution of water environment by pesticides, particularly herbicides, has been recognised in agricultural areas of the world for many years, and considerable evidence has been accumulated to suggest that many water resources are contaminated with organic pesticides (Cohen, 1986; Chiron et al., 1993).
* Corresponding author. Tel.: +34 950 015649; fax: +34 950 015008. E-mail address: [email protected] (M.D. Uren˜a-Amate).
Triazines are used widely as herbicides and atrazine, despite being banned in most European countries, atrazine is the most widely used herbicide in the United States and is registered in more than 70 countries worldwide (Rippen, 1992; Kauffmann et al., 2000). Furthermore, atrazine has been identified as a potential leacher by using the ground water ubiquity score modelling technique (Gustafson, 1989), and it has been widely detected in ground water (Parrilla et al., 1994; Chiron et al., 1995), so it is potentially toxic to both humans and ecosystems. A new class of kerolite/stevensite interstratified material has become of increasing interest because of its
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M.D. Uren˜a-Amate et al. / Chemosphere 59 (2005) 69–74
applications as a bleaching material and adsorbent of contaminant organic molecules (Pozo et al., 1999; Gonza´lez-Pradas et al., 2000; Gonza´lez-Pradas et al., 2003; Socı´as-Viciana et al., 2003). The kerolite/stevensite here cited belongs to the Madrid Basin and is a keroliterich material (80% of kerolite), (denoted kerolite in the text), with a structure similar to talc, although more disordered and with a higher degree of hydration (Brindley et al., 1977; Zelazny and White, 1989). Given that adsorption on activated carbon or clay is a method used for decontaminating organic polluted waters, (Hutson and Roberts, 1990; Svoboda et al., 1991), we have studied the adsorption process of atrazine from aqueous solution on the above indicated kerolitic material. In addition, several studies have shown that factors such as the temperature or the presence of ionic strength affects considerably the adsorption of certain pesticides such as atrazine (Hurle and Freed, 1972; Fruhstorfer et al., 1993). Taking into account the above, and according to our previous research on the use of heat treated kerolites for removing atrazine from water, we have considered it useful to study the adsorption process of atrazine from both, pure water and 0.1 M KCl solutions at work temperatures of 10 C, 25 C and 40 C, using a heat treated kerolite sample at 600 C (Gonza´lez-Pradas et al., 2003). The main objective of the present research was to evaluate the effects of temperature and the presence of one electrolyte in the medium (0.1 M KCl) for removing atrazine from aqueous solution by using kerolite activated at 600 C.
2. Materials and methods The clay used as adsorbent for this series of experiments was a 600 C heat-treated kerolite from the Madrid Basin (Spain) (crushed to a particle size less than 150 lm), previously studied by other authors (Pozo et al., 1999). Heat activation of the natural kerolite was carried out by heating it at 600 C for 24 h. The material obtained was analysed chemically by X-ray fluorescence spectroscopy with a Philips PW 1404 instrument. Specific surface area and micropore volume were determined from N2 adsorption isotherms at 77.4 K, in a volumetric adsorption system, Gemini II-2375 (Micromeritics). The nitrogen used was 99.998% pure. The sample was degassed previously at 110 C for 24 h. Further information on the pore texture of the material used, including contribution of the meso- and macropores to the total pore surface area and volume, was obtained using an Autopore III porosimeter (Micromeritics), with maximum pressures of 413685.440 kPa that covers spectra between 1.8 and 10 000 nm. The system
incorporates a data processor which facilitates analysis of the experimental extrusion and intrusion curves of mercury pore size distribution, accumulative volume and equivalent surface area of pores. Thermogravimetry (TGA and DTG) curves were obtained with a Mettler TA 3000 thermogravimetric analysis unit and Mettler TG 50 thermobalance. The FTIR spectrum of the sample was recorded using KBr pellets on an ATI Mattson spectrometer over a range of 4000–400 cm1. The XRD pattern was obtained on a Philips PW-1700 diffractometer using graphite-monochromated Cu Ka radiation. All these results have been previously published by the present authors (Socias-Viciana et al., 2003). Table 1 shows the main properties of the heat-treated kerolite sample at 600 C. Analytically pure atrazine was obtained from Laboratory Dr. Ehrenstorfer-Scha¨fers (Augsburg, Germany) and used as adsorbate in this study. Sorption isotherms were determined by batch equilibration of 0.25 g of the clay sample with 0.1 l of pure water and 0.1 M KCl solutions of atrazine of varied initial concentration (1.68–29.5 mg l1), this range being selected in order to cover a wide range of atrazine concentrations. Experiments were carried out in a thermostatic shaker bath at 10 C, 25 C and 40 C. Preliminary experiments were conducted for various time intervals to determine when sorption equilibrium was reached. The sorption equilibrium time required for atrazine was 24 h. After equilibration, the suspensions were centrifuged, the supernatant filtered through 0.45 lm nylon filters and the pesticide concentration measured in the supernatant solution (Ce) by high performance liquid chromatography (HPLC) using a diode array detector and data station. Separation by isocratic elution was performed on a 150 mm · 3.9 mm id Waters Nova Pack bondedphase column (C18 reversed-phase, particle size, 4 lm). The mobile phase used as eluent was an (60:40) acetonitrile-water mix and the pesticide analysed at 222 nm, its wavelength of maximum absorption. The amount adsorbed (X) was calculated from the difference in concentration between initial (Ci) and final or equilibrium (Ce) solution. Blanks containing no atrazine, and two replicates of each adsorption point were used for each series of experiments. The pH of the blanks as well as those corresponding to the several solutions in contact with the kerolite used as adsorbent, were measured using a pH-meter Crison, model pH 2002. Solubility of atrazine in pure water and in 0.1 M KCl solutions at 10 C, 25 C and 40 C, was also determined by equilibrating in a thermostatic shaker bath at each temperature, saturated solutions of atrazine with analytically pure atrazine. Once the highest concentration of the pesticide—measured by HPLC as above indicated—had been reached, and maintaining it constant for 2 weeks, the equilibrium of the dissolution process was supposed
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Table 1 Main properties of the heat-treated kerolite sample at 600 C SBETa (m2 g1)
Vmicropores (cm3 g1)
Vmesopores (cm3 g1)
Vmacropores (cm3 g1)
Weight loss (%) 35–400 C
Weight loss (%) 400–850 C
Total weight loss (%)
224
0.026
0.27
0.64
2.48
4.16
6.64
Chemical analysis of the kerolite sample. Percentage of major oxides SiO2 Al2O3 Fe2O3 CaO TiO2 MnO
K2O
MgO
Na2O
P2O5
L.O.I.b
58.85
0.666
27.89
0.081
0.025
7.98
a b
2.40
0.976
0.859
0.158
0.024
Correlation coefficients (r) were 0.9999 in all cases and significant at 0.001 probability level. Lost on ignition.
completed, and the solubility determined. Three replicates were used for each series of experiments.
8 6
3. Results and discussion
4
Table 2 shows the solubility of atrazine, at 10 C, 25 C and 40 C, in both, pure water and 0.1 M KCl solutions. As can be seen from Table 2, the solubility of atrazine increases from 30.2 mg l1 at 10 C up to 41.0 mg l1 at 40 C (pure water solution); the presence of ionic strength decreasing the solubility (from 30.2 mg l1 to 27.1 mg l1 at 10 C and from 41.0 mg l1 to 35.1 mg l1 at 40 C). The pH values of the blanks containing no atrazine were 7.5 and 7.3 in pure water solution and in 0.1 M KCl solution, respectively. No significantly differences relating these values were observed for the measured pH of the several solutions of atrazine in contact with the kerolite sample. We can then suppose that atrazine is adsorbed on the kerolite surface as non-ionic species fundamentally, the adsorption mechanism possibly taking place by cation bridging which involves the formation of an inner-sphere complex between the kerolite Mg2+ cations and the atrazine –NH group, as suggested by the present authors in a previous paper (Socı´as-Viciana et al., 2003). Fig. 1 shows the adsorption isotherms of atrazine on the heat-treated kerolite sample at 10 C, 25 C and 40 C in pure water solution. According to the slope of the initial portion of the curves, these isotherms may be classified, in general, as L-type of the Giles classification
Table 2 Solubility of atrazine in pure water and saline medium Medium
Pure water medium 0.1 M KCl medium
Solubility (mg l1) 10 C
25 C
40 C
30.2 27.1
34.5 30.3
41.0 35.1
10ºC 25ºC
2
40ºC
0
0
5
10
15
20
25
30
Fig. 1. Adsorption isotherms at 10 C, 25 C and 40 C for atrazine on the 600 C heat-treated kerolite sample in pure water medium.
(Giles et al., 1960). This suggests that kerolite has an intermediate affinity for atrazine and that no strong competition from the solvent for adsorption sites occurs. Nevertheless, an increase of the slopes of the initial portion of each curve can be noted in Fig. 1 as temperature decreases, mainly from 40 C to 25 C and 10 C. This indicates a decreased affinity of the active sites for atrazine as temperature increases from 10 C up to 40 C. It is also noteworthy in Fig. 1 that for a given Ce, the amount of atrazine adsorbed (X) is higher as temperature decreases from 40 C up to 10 C. On the other hand, in order to analyse the influence of the presence of 0.1 M KCl solution in the medium, Fig. 2 shows together, the adsorption isotherms of atrazine on the kerolite sample from both, pure water solution and 0.1 M KCl solution, at 10 C, 25 C and 40 C. As can be seen, the shape of the isotherms from 0.1 M KCl solution is the same shape as that obtained for pure water solution. Nevertheless, it is important to note that for a given Ce, the amount of atrazine adsorbed is higher when 0.1 M KCl solution is used in the adsorption experiments; the higher the temperature, the more pronounced effect of the ionic strength. To evaluate the adsorption capacities of the 600 C heat treated kerolite at 10 C, 25 C and 40 C, in pure
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(Adamson, 1982) and Henry (Voice et al., 1983). The best results were obtained using the Langmuir and Freundlich equations. The linear form of the Langmuir equation is [1]:
14 10ºC
3 -1 X.10 (mg kg )
12
(a)
10 8
Ce 1 Ce ¼ þ X ðb X m Þ X m
6 4
Pure water medium Saline medium (0.1 M KCl)
2 0 14
3
-1
X.10 (mg kg )
(b)
25ºC
12 10 8 6 4 2 0
3
-1
X.10 (mg kg )
14 (c)
40ºC
12 10 8 6 4 2 0 0
5
10
15
20
25
30
Fig. 2. Compared adsorption isotherms for atrazine in pure water and 0.1 M KCl solutions at (a) 10 C, (b) 25 C and (c) 40 C on the 600 C heat-treated kerolite sample.
water and in 0.1 M KCl solutions, experimental data points were fitted to several equations applicable to adsorption from solution processes such as Langmuir (Kipling, 1980), B.E.T. (Tiren, 1982), Freundlich
ð1Þ
where X = pesticide adsorbed per kg of adsorbent, mg kg1; Xm = maximum amount of pesticide that can be adsorbed in a monolayer (adsorption capacity), mg kg1; b = constant relating to the energy of adsorption, l mg1. Langmuir parameters (Xm, b) are summarised, together with the correlation coefficients, (all correlation significant at the 0.001 probability level), in Table 3. Xm values range from 2.3 · 103 mg kg1 (pure water solution at 40 C) up to 15.2 · 103 mg kg1 (0.1 M KCl solution at 10 C). The most prominent feature of these results is the difference observed among the Xm values at the different temperatures studied. Increasing temperature from 10 C to 40 C results, mainly for adsorption from pure water solution, in a clear decrease in the amount of atrazine adsorbed. The decrease of the Xm values might be explained assuming that as temperature increases, the formation of the unions between the kerolite surface and the atrazine molecules will be diminish, according to the adsorption mechanism previously indicated. This trend has been also reported previously by other authors for the sorption of atrazine on kaolinitics and montmorillonitic clays (Fruhstorfer et al., 1993). It is noteworthy that the Xm values indicated in Table 3 are much higher than those reported in the literature for the same herbicide but using different adsorbents like peat, humic acids (Gonza´lez-Pradas et al., 1996), or other clays like sepiolite (Gonza´lez-Pradas et al., 1999) and bentonite (Gonza´lez-Pradas et al., 1997). The effect of the electrolyte on the amount of compound adsorbed is also presented in Table 3. As can be seen from the Xm values, the effect of the presence of 0.1 M KCl does not seem clearly defined, since
Table 3 Parameters of the Langmuir equation for adsorption of atrazine on the 600 C heat-treated kerolite sample Temperature
Xm · 103 (mg kg1)
b (l mg1)
r
R (% pesticide removed)
Pure water medium 10 C 25 C 40 C
11.1 8.7 2.3
0.241 0.342 0.370
0.9837 0.9966 0.9847
89 (89.52–90.52)a 78 (77.25–78.75) 39 (38.15–39.65)
Saline medium (0.1 M KCl) 10 C 15.2 25 C 8.8 40 C 8.8
0.173 0.758 0.405
0.9887 0.9957 0.9957
93 (92.25–93.75) 78 (77.05–78.55) 78 (77.35–78.85)
a
Values in parentheses represent the standard deviation.
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although at 10 C and 40 C the adsorption of atrazine by the heat-treated kerolite sample is higher than that obtained in non saline medium (15.2 · 103 vs. 11.1 · 103 mg kg1 at 10 C; and 8.8 · 103 vs. 2.3 · 103 mg kg1 at 40 C), there is not a significant difference at 25 C. In order to clarify the influence of the presence of 0.1 M KCl in the medium at 25 C, the experimental adsorption points were fitted to a more empirical equation such as the Freundlich one, minimising in this way the possible deviations derived from the most theoretical aspect of the Langmuir equation. The linear form of the Freundlich equation is [2]: Log X ¼ Log K f þ n Log C e
ð2Þ
where X = atrazine adsorbed per kilogram of kerolite, mg kg1; Kf = the value of X at concentration equal to 1 mg l1, mg kg1; n = slope of the isotherms. Both, Kf and n values are constants being indicative of the extent of adsorption and the degree of non-linearity between solution concentration and adsorption, respectively. Table 4 shows the Kf and n values calculated by the least square method applied to the straight lines obtained from Eq. (2). The correlation coefficients were in all cases greater than 0.98 (all correlation were significant at the 0.001 probability level). As Kf represents the amount of atrazine retained at Ce = 1 mg l1, the authors have considered useful for comparative purpose, to define a new parameter, K10, which represents the amount of pesticide adsorbed at a higher equilibrium concentration (Ce = 10 mg l1). This parameter which also appears in Table 4—is more comparable to the Xm parameter obtained from the theoretical Langmuir equation and it is much more significant for a practical purpose. As can be seen from Table 4, the behaviour of K10 values is similar to that obtained for Xm, so decreasing from 8.01 · 103 mg kg1 at 10 C up to 1.75 · 103 mg kg1 at 40 C in pure water medium and from 8.81 · 103 mg kg1 at 10 C up to 7.13 · 103 mg kg1 at 40 C in saline medium. Thus, the effect of ionic strength appears now clearly defined, Table 4 Parameters of the Freundlich equation and K10 for adsorption of atrazine on the 600 C heat-treated kerolite sample n
r
K10 · 103 (mg kg1)
Pure water medium 10 C 2.19 25 C 2.31 40 C 0.67
0.56 0.44 0.41
0.9972 0.9924 0.9958
8.01 6.34 1.75
Saline medium (0.1 M KCl) 10 C 2.45 25 C 3.31 40 C 2.38
0.55 0.40 0.48
0.9956 0.9845 0.9914
8.81 8.25 7.13
Temperature
Kf · 103 (mg kg1)
73
the parameter K10 being higher in 0.1 M KCl than in pure water solution. This increased adsorption of atrazine is probably due to the decrease of the solubility of this herbicide as indicated in Table 2. The decrease in solubility of atrazine in salt solution can be simply characterized as a ‘‘salting out’’ effect, since the salt ions attract around themselves the polarizable water molecules, making the solution more polar and reducing the amount of water molecules available (Hurle and Freed, 1972). The removal efficiency, R, was calculated using a 15 mg l1 aqueous solution of atrazine (0.1 l), placed in a stoppered conical flask and shaken for 24 h with an amount (0.25 g) of the samples. R was calculated from the difference between the initial and equilibrium solution concentration and it is expressed in terms of percentages also in Table 3. As can be seen from Table 3, the removal efficiency values (R) range from 39% (40 C, pure water solution) up to 93% (10 C, 0.1 M KCl solution), these values showing, as expected, a similar variation to that reported for the Xm values. 4. Conclusions These experiments indicate that the lower temperature, the more effective the adsorption of atrazine from both, pure water and 0.1 M KCl solutions. The presence of 0.1 M KCl in the medium tends also to increase the adsorption of atrazine in the range of temperature studied. The results obtained from this work could be of interest by showing the better experimental conditions for the potential use of the 600 C heat-activated kerolite for adsorption of atrazine. It seems, in general, that by conducting the adsorption process at 10 C in presence of 0.1 M KCl solution for 24 h, the optimum adsorption of atrazine could be achieved. As this type of clay is relatively plentiful, this activated sample might be reasonably used in order to remove atrazine from aqueous solutions, in the experimental conditions carried out in this paper.
Acknowledgment ¨ DCHEMIE for the kerolite samples. We thank SU
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Gustafson, D.I., 1989. Groundwater ubiquity score: a simple method for assessing pesticide leachability. Environ. Toxicol. Chem. 8, 339–357. Hurle, K.B., Freed, V.H., 1972. Effect of electrolytes on the solubility of some 1,3,5-triazines and substituted ureas and their adsorption on soil. Weed Res. 12, 1–10. Hutson, D.H., Roberts, T.R., 1990. Environmental Fate of Pesticides. Wiley, New York. Kauffmann, C.O., Shoseyov, Shpigel, E., Bayer, E.A., Lamed, R., Shoham, Y., Mandelbaum, R.T., 2000. Novel methodology for enzymatic removal of atrazine from water by CBD-Fusion protein immobilized on cellulose. Environ. Sci. Technol. 34, 1292–1296. Kipling, J.J., 1980. Adsorption from Solutions of non-electrolytes. Ed. Academic Press Inc., London. Parrilla, P., Martı´nez-Vidal, J.L., Martı´nez-Galera, M., Freniche, A.G., 1994. Simple and rapid screening procedure for pesticides in water using SPE and HPLC/DAD detection. Fresenius J. Anal. Chem. 350, 633–637. Pozo, M., Casas, J., Martı´n de Vidales, J.L., Medina, J.A., Martı´n Rubı´, J.A., 1999. Caracterı´sticas texturales y composicionales en depo´sitos de arcillas magne´sicas de la Cuenca de Madrid. I Kerolitas (sector de Esquivias y Pinto). Boletı´n Geolo´gico Minero 110 (1), 77–102. Rippen, G., 1992. Environmental chemical products handbook, third ed. Ecomed, Landsberg and Lech, Germany. Socı´as-Viciana, M., Gonza´lez-Pradas, E., Saifi, M., Uren˜aAmate, M.D., Flores-Ce´spedes, F., Ferna´ndez-Pe´rez, M., 2003. The potential removal of imidacloprid from water by heat treated kerolites. Pest Manage. Sci. 59, 1162–1168. Svoboda, L., Jandera, P., Chura´cek, J., 1991. Study of the sorption of selected pesticides on nonpolar sorbents. Collect Czech Cem. Commun. 56, 317–333. Tiren, G., 1982. The application of the modified BET equation to multimolecular adsorption from solution. J. Colloid Interface Sci. 85, 601–603. Voice, T.C., Rice, C.P., Weber, W.J., 1983. Effect of solids concentration on the sorptive partitioning of hydrophobic pollutants in aquatic systems. Environ. Sci. Technol. 17, 513–518. Zelazny, L.W., White, G.N., 1989. The pyrophyllite-talc group. In: Minerals in Soil Environments, second ed. In: Dixon, J.B., Weed, S.B. (Eds.), SSSA Book Series, no. 1 Madison, WI, USA, pp. 527–550.
Effects of ionic strength and temperature on adsorption of atrazine by a heat treated kerolite M.D. Uren˜a-Amate *, M. Socı´as-Viciana, E. Gonza´lez-Pradas, M. Saifi Department of Inorganic Chemistry, University of Almerı´a, La Can˜ada San Urbano s/n, 04120, Almerı´a, Spain Received 19 January 2004; received in revised form 15 September 2004; accepted 29 September 2004
Abstract The adsorption of 6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (atrazine) on a heat treated kerolite sample at 600 C (K-600) from pure water solution at 10 C, 25 C and 40 C has been studied. The influence of the presence of 0.1 M KCl in the medium was also investigated for a better understanding of variables affecting the adsorption of this herbicide. The experimental adsorption data points were fitted to the Langmuir equation in order to calculate the adsorption capacities (Xm) of the samples; Xm values range from 2.3 · 103 mg kg1 (pure water solution at 40 C) up to 15.2 · 103 mg kg1 (0.1 M KCl solution at 10 C). The adsorption data were also fitted to the Freundlich equation in order to clarify the influence of the presence of 0.1 M KCl on atrazine adsorption. The parameter K10 obtained from this equation (adsorption capacity at an equilibrium solution concentration of atrazine equal to 10 mg l1) shows clearly that the presence of 0.1 M KCl in the medium tends to increase the adsorption of atrazine in the range of temperature studied. The adsorption experiment also showed that the lower temperature, the more effective the adsorption of atrazine from both, pure water and 0.1 M KCl solutions. The values of the removal efficiency (R) obtained ranged from 39% at 40 C (pure water solution) up to 93% at 10 C (0.1 M KCl solution). 2004 Elsevier Ltd. All rights reserved. Keywords: Kerolite; Adsorption; Atrazine
1. Introduction Pollution of water environment by pesticides, particularly herbicides, has been recognised in agricultural areas of the world for many years, and considerable evidence has been accumulated to suggest that many water resources are contaminated with organic pesticides (Cohen, 1986; Chiron et al., 1993).
* Corresponding author. Tel.: +34 950 015649; fax: +34 950 015008. E-mail address: [email protected] (M.D. Uren˜a-Amate).
Triazines are used widely as herbicides and atrazine, despite being banned in most European countries, atrazine is the most widely used herbicide in the United States and is registered in more than 70 countries worldwide (Rippen, 1992; Kauffmann et al., 2000). Furthermore, atrazine has been identified as a potential leacher by using the ground water ubiquity score modelling technique (Gustafson, 1989), and it has been widely detected in ground water (Parrilla et al., 1994; Chiron et al., 1995), so it is potentially toxic to both humans and ecosystems. A new class of kerolite/stevensite interstratified material has become of increasing interest because of its
0045-6535/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2004.09.098
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applications as a bleaching material and adsorbent of contaminant organic molecules (Pozo et al., 1999; Gonza´lez-Pradas et al., 2000; Gonza´lez-Pradas et al., 2003; Socı´as-Viciana et al., 2003). The kerolite/stevensite here cited belongs to the Madrid Basin and is a keroliterich material (80% of kerolite), (denoted kerolite in the text), with a structure similar to talc, although more disordered and with a higher degree of hydration (Brindley et al., 1977; Zelazny and White, 1989). Given that adsorption on activated carbon or clay is a method used for decontaminating organic polluted waters, (Hutson and Roberts, 1990; Svoboda et al., 1991), we have studied the adsorption process of atrazine from aqueous solution on the above indicated kerolitic material. In addition, several studies have shown that factors such as the temperature or the presence of ionic strength affects considerably the adsorption of certain pesticides such as atrazine (Hurle and Freed, 1972; Fruhstorfer et al., 1993). Taking into account the above, and according to our previous research on the use of heat treated kerolites for removing atrazine from water, we have considered it useful to study the adsorption process of atrazine from both, pure water and 0.1 M KCl solutions at work temperatures of 10 C, 25 C and 40 C, using a heat treated kerolite sample at 600 C (Gonza´lez-Pradas et al., 2003). The main objective of the present research was to evaluate the effects of temperature and the presence of one electrolyte in the medium (0.1 M KCl) for removing atrazine from aqueous solution by using kerolite activated at 600 C.
2. Materials and methods The clay used as adsorbent for this series of experiments was a 600 C heat-treated kerolite from the Madrid Basin (Spain) (crushed to a particle size less than 150 lm), previously studied by other authors (Pozo et al., 1999). Heat activation of the natural kerolite was carried out by heating it at 600 C for 24 h. The material obtained was analysed chemically by X-ray fluorescence spectroscopy with a Philips PW 1404 instrument. Specific surface area and micropore volume were determined from N2 adsorption isotherms at 77.4 K, in a volumetric adsorption system, Gemini II-2375 (Micromeritics). The nitrogen used was 99.998% pure. The sample was degassed previously at 110 C for 24 h. Further information on the pore texture of the material used, including contribution of the meso- and macropores to the total pore surface area and volume, was obtained using an Autopore III porosimeter (Micromeritics), with maximum pressures of 413685.440 kPa that covers spectra between 1.8 and 10 000 nm. The system
incorporates a data processor which facilitates analysis of the experimental extrusion and intrusion curves of mercury pore size distribution, accumulative volume and equivalent surface area of pores. Thermogravimetry (TGA and DTG) curves were obtained with a Mettler TA 3000 thermogravimetric analysis unit and Mettler TG 50 thermobalance. The FTIR spectrum of the sample was recorded using KBr pellets on an ATI Mattson spectrometer over a range of 4000–400 cm1. The XRD pattern was obtained on a Philips PW-1700 diffractometer using graphite-monochromated Cu Ka radiation. All these results have been previously published by the present authors (Socias-Viciana et al., 2003). Table 1 shows the main properties of the heat-treated kerolite sample at 600 C. Analytically pure atrazine was obtained from Laboratory Dr. Ehrenstorfer-Scha¨fers (Augsburg, Germany) and used as adsorbate in this study. Sorption isotherms were determined by batch equilibration of 0.25 g of the clay sample with 0.1 l of pure water and 0.1 M KCl solutions of atrazine of varied initial concentration (1.68–29.5 mg l1), this range being selected in order to cover a wide range of atrazine concentrations. Experiments were carried out in a thermostatic shaker bath at 10 C, 25 C and 40 C. Preliminary experiments were conducted for various time intervals to determine when sorption equilibrium was reached. The sorption equilibrium time required for atrazine was 24 h. After equilibration, the suspensions were centrifuged, the supernatant filtered through 0.45 lm nylon filters and the pesticide concentration measured in the supernatant solution (Ce) by high performance liquid chromatography (HPLC) using a diode array detector and data station. Separation by isocratic elution was performed on a 150 mm · 3.9 mm id Waters Nova Pack bondedphase column (C18 reversed-phase, particle size, 4 lm). The mobile phase used as eluent was an (60:40) acetonitrile-water mix and the pesticide analysed at 222 nm, its wavelength of maximum absorption. The amount adsorbed (X) was calculated from the difference in concentration between initial (Ci) and final or equilibrium (Ce) solution. Blanks containing no atrazine, and two replicates of each adsorption point were used for each series of experiments. The pH of the blanks as well as those corresponding to the several solutions in contact with the kerolite used as adsorbent, were measured using a pH-meter Crison, model pH 2002. Solubility of atrazine in pure water and in 0.1 M KCl solutions at 10 C, 25 C and 40 C, was also determined by equilibrating in a thermostatic shaker bath at each temperature, saturated solutions of atrazine with analytically pure atrazine. Once the highest concentration of the pesticide—measured by HPLC as above indicated—had been reached, and maintaining it constant for 2 weeks, the equilibrium of the dissolution process was supposed
M.D. Uren˜a-Amate et al. / Chemosphere 59 (2005) 69–74
71
Table 1 Main properties of the heat-treated kerolite sample at 600 C SBETa (m2 g1)
Vmicropores (cm3 g1)
Vmesopores (cm3 g1)
Vmacropores (cm3 g1)
Weight loss (%) 35–400 C
Weight loss (%) 400–850 C
Total weight loss (%)
224
0.026
0.27
0.64
2.48
4.16
6.64
Chemical analysis of the kerolite sample. Percentage of major oxides SiO2 Al2O3 Fe2O3 CaO TiO2 MnO
K2O
MgO
Na2O
P2O5
L.O.I.b
58.85
0.666
27.89
0.081
0.025
7.98
a b
2.40
0.976
0.859
0.158
0.024
Correlation coefficients (r) were 0.9999 in all cases and significant at 0.001 probability level. Lost on ignition.
completed, and the solubility determined. Three replicates were used for each series of experiments.
8 6
3. Results and discussion
4
Table 2 shows the solubility of atrazine, at 10 C, 25 C and 40 C, in both, pure water and 0.1 M KCl solutions. As can be seen from Table 2, the solubility of atrazine increases from 30.2 mg l1 at 10 C up to 41.0 mg l1 at 40 C (pure water solution); the presence of ionic strength decreasing the solubility (from 30.2 mg l1 to 27.1 mg l1 at 10 C and from 41.0 mg l1 to 35.1 mg l1 at 40 C). The pH values of the blanks containing no atrazine were 7.5 and 7.3 in pure water solution and in 0.1 M KCl solution, respectively. No significantly differences relating these values were observed for the measured pH of the several solutions of atrazine in contact with the kerolite sample. We can then suppose that atrazine is adsorbed on the kerolite surface as non-ionic species fundamentally, the adsorption mechanism possibly taking place by cation bridging which involves the formation of an inner-sphere complex between the kerolite Mg2+ cations and the atrazine –NH group, as suggested by the present authors in a previous paper (Socı´as-Viciana et al., 2003). Fig. 1 shows the adsorption isotherms of atrazine on the heat-treated kerolite sample at 10 C, 25 C and 40 C in pure water solution. According to the slope of the initial portion of the curves, these isotherms may be classified, in general, as L-type of the Giles classification
Table 2 Solubility of atrazine in pure water and saline medium Medium
Pure water medium 0.1 M KCl medium
Solubility (mg l1) 10 C
25 C
40 C
30.2 27.1
34.5 30.3
41.0 35.1
10ºC 25ºC
2
40ºC
0
0
5
10
15
20
25
30
Fig. 1. Adsorption isotherms at 10 C, 25 C and 40 C for atrazine on the 600 C heat-treated kerolite sample in pure water medium.
(Giles et al., 1960). This suggests that kerolite has an intermediate affinity for atrazine and that no strong competition from the solvent for adsorption sites occurs. Nevertheless, an increase of the slopes of the initial portion of each curve can be noted in Fig. 1 as temperature decreases, mainly from 40 C to 25 C and 10 C. This indicates a decreased affinity of the active sites for atrazine as temperature increases from 10 C up to 40 C. It is also noteworthy in Fig. 1 that for a given Ce, the amount of atrazine adsorbed (X) is higher as temperature decreases from 40 C up to 10 C. On the other hand, in order to analyse the influence of the presence of 0.1 M KCl solution in the medium, Fig. 2 shows together, the adsorption isotherms of atrazine on the kerolite sample from both, pure water solution and 0.1 M KCl solution, at 10 C, 25 C and 40 C. As can be seen, the shape of the isotherms from 0.1 M KCl solution is the same shape as that obtained for pure water solution. Nevertheless, it is important to note that for a given Ce, the amount of atrazine adsorbed is higher when 0.1 M KCl solution is used in the adsorption experiments; the higher the temperature, the more pronounced effect of the ionic strength. To evaluate the adsorption capacities of the 600 C heat treated kerolite at 10 C, 25 C and 40 C, in pure
72
M.D. Uren˜a-Amate et al. / Chemosphere 59 (2005) 69–74
(Adamson, 1982) and Henry (Voice et al., 1983). The best results were obtained using the Langmuir and Freundlich equations. The linear form of the Langmuir equation is [1]:
14 10ºC
3 -1 X.10 (mg kg )
12
(a)
10 8
Ce 1 Ce ¼ þ X ðb X m Þ X m
6 4
Pure water medium Saline medium (0.1 M KCl)
2 0 14
3
-1
X.10 (mg kg )
(b)
25ºC
12 10 8 6 4 2 0
3
-1
X.10 (mg kg )
14 (c)
40ºC
12 10 8 6 4 2 0 0
5
10
15
20
25
30
Fig. 2. Compared adsorption isotherms for atrazine in pure water and 0.1 M KCl solutions at (a) 10 C, (b) 25 C and (c) 40 C on the 600 C heat-treated kerolite sample.
water and in 0.1 M KCl solutions, experimental data points were fitted to several equations applicable to adsorption from solution processes such as Langmuir (Kipling, 1980), B.E.T. (Tiren, 1982), Freundlich
ð1Þ
where X = pesticide adsorbed per kg of adsorbent, mg kg1; Xm = maximum amount of pesticide that can be adsorbed in a monolayer (adsorption capacity), mg kg1; b = constant relating to the energy of adsorption, l mg1. Langmuir parameters (Xm, b) are summarised, together with the correlation coefficients, (all correlation significant at the 0.001 probability level), in Table 3. Xm values range from 2.3 · 103 mg kg1 (pure water solution at 40 C) up to 15.2 · 103 mg kg1 (0.1 M KCl solution at 10 C). The most prominent feature of these results is the difference observed among the Xm values at the different temperatures studied. Increasing temperature from 10 C to 40 C results, mainly for adsorption from pure water solution, in a clear decrease in the amount of atrazine adsorbed. The decrease of the Xm values might be explained assuming that as temperature increases, the formation of the unions between the kerolite surface and the atrazine molecules will be diminish, according to the adsorption mechanism previously indicated. This trend has been also reported previously by other authors for the sorption of atrazine on kaolinitics and montmorillonitic clays (Fruhstorfer et al., 1993). It is noteworthy that the Xm values indicated in Table 3 are much higher than those reported in the literature for the same herbicide but using different adsorbents like peat, humic acids (Gonza´lez-Pradas et al., 1996), or other clays like sepiolite (Gonza´lez-Pradas et al., 1999) and bentonite (Gonza´lez-Pradas et al., 1997). The effect of the electrolyte on the amount of compound adsorbed is also presented in Table 3. As can be seen from the Xm values, the effect of the presence of 0.1 M KCl does not seem clearly defined, since
Table 3 Parameters of the Langmuir equation for adsorption of atrazine on the 600 C heat-treated kerolite sample Temperature
Xm · 103 (mg kg1)
b (l mg1)
r
R (% pesticide removed)
Pure water medium 10 C 25 C 40 C
11.1 8.7 2.3
0.241 0.342 0.370
0.9837 0.9966 0.9847
89 (89.52–90.52)a 78 (77.25–78.75) 39 (38.15–39.65)
Saline medium (0.1 M KCl) 10 C 15.2 25 C 8.8 40 C 8.8
0.173 0.758 0.405
0.9887 0.9957 0.9957
93 (92.25–93.75) 78 (77.05–78.55) 78 (77.35–78.85)
a
Values in parentheses represent the standard deviation.
M.D. Uren˜a-Amate et al. / Chemosphere 59 (2005) 69–74
although at 10 C and 40 C the adsorption of atrazine by the heat-treated kerolite sample is higher than that obtained in non saline medium (15.2 · 103 vs. 11.1 · 103 mg kg1 at 10 C; and 8.8 · 103 vs. 2.3 · 103 mg kg1 at 40 C), there is not a significant difference at 25 C. In order to clarify the influence of the presence of 0.1 M KCl in the medium at 25 C, the experimental adsorption points were fitted to a more empirical equation such as the Freundlich one, minimising in this way the possible deviations derived from the most theoretical aspect of the Langmuir equation. The linear form of the Freundlich equation is [2]: Log X ¼ Log K f þ n Log C e
ð2Þ
where X = atrazine adsorbed per kilogram of kerolite, mg kg1; Kf = the value of X at concentration equal to 1 mg l1, mg kg1; n = slope of the isotherms. Both, Kf and n values are constants being indicative of the extent of adsorption and the degree of non-linearity between solution concentration and adsorption, respectively. Table 4 shows the Kf and n values calculated by the least square method applied to the straight lines obtained from Eq. (2). The correlation coefficients were in all cases greater than 0.98 (all correlation were significant at the 0.001 probability level). As Kf represents the amount of atrazine retained at Ce = 1 mg l1, the authors have considered useful for comparative purpose, to define a new parameter, K10, which represents the amount of pesticide adsorbed at a higher equilibrium concentration (Ce = 10 mg l1). This parameter which also appears in Table 4—is more comparable to the Xm parameter obtained from the theoretical Langmuir equation and it is much more significant for a practical purpose. As can be seen from Table 4, the behaviour of K10 values is similar to that obtained for Xm, so decreasing from 8.01 · 103 mg kg1 at 10 C up to 1.75 · 103 mg kg1 at 40 C in pure water medium and from 8.81 · 103 mg kg1 at 10 C up to 7.13 · 103 mg kg1 at 40 C in saline medium. Thus, the effect of ionic strength appears now clearly defined, Table 4 Parameters of the Freundlich equation and K10 for adsorption of atrazine on the 600 C heat-treated kerolite sample n
r
K10 · 103 (mg kg1)
Pure water medium 10 C 2.19 25 C 2.31 40 C 0.67
0.56 0.44 0.41
0.9972 0.9924 0.9958
8.01 6.34 1.75
Saline medium (0.1 M KCl) 10 C 2.45 25 C 3.31 40 C 2.38
0.55 0.40 0.48
0.9956 0.9845 0.9914
8.81 8.25 7.13
Temperature
Kf · 103 (mg kg1)
73
the parameter K10 being higher in 0.1 M KCl than in pure water solution. This increased adsorption of atrazine is probably due to the decrease of the solubility of this herbicide as indicated in Table 2. The decrease in solubility of atrazine in salt solution can be simply characterized as a ‘‘salting out’’ effect, since the salt ions attract around themselves the polarizable water molecules, making the solution more polar and reducing the amount of water molecules available (Hurle and Freed, 1972). The removal efficiency, R, was calculated using a 15 mg l1 aqueous solution of atrazine (0.1 l), placed in a stoppered conical flask and shaken for 24 h with an amount (0.25 g) of the samples. R was calculated from the difference between the initial and equilibrium solution concentration and it is expressed in terms of percentages also in Table 3. As can be seen from Table 3, the removal efficiency values (R) range from 39% (40 C, pure water solution) up to 93% (10 C, 0.1 M KCl solution), these values showing, as expected, a similar variation to that reported for the Xm values. 4. Conclusions These experiments indicate that the lower temperature, the more effective the adsorption of atrazine from both, pure water and 0.1 M KCl solutions. The presence of 0.1 M KCl in the medium tends also to increase the adsorption of atrazine in the range of temperature studied. The results obtained from this work could be of interest by showing the better experimental conditions for the potential use of the 600 C heat-activated kerolite for adsorption of atrazine. It seems, in general, that by conducting the adsorption process at 10 C in presence of 0.1 M KCl solution for 24 h, the optimum adsorption of atrazine could be achieved. As this type of clay is relatively plentiful, this activated sample might be reasonably used in order to remove atrazine from aqueous solutions, in the experimental conditions carried out in this paper.
Acknowledgment ¨ DCHEMIE for the kerolite samples. We thank SU
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