Arch. Environ. Contam. Toxicol. 50, 128–137 (2006) DOI: 10.1007/s00244-005-7036-3
Surfactant-Enhanced Desorption of Atrazine and Linuron Residues as Affected by Aging of Herbicides in Soil M. S. Rodriguez-Cruz, M. J. Sanchez-Martin,* M. Sanchez-Camazano Instituto de Recursos Naturales y Agrobiologa, CSIC. Apdo. 257, 37071 Salamanca, Spain
Received: 14 February 2005 /Accepted: 23 May 2005
Abstract. In the present work, we studied the efficiency of two surfactants, one anionic (SDS) and other non-ionic (Triton X100), in the desorption of atrazine and linuron after 0, 3, and 9 months of soil-herbicide aging time. Batch desorption studies were conducted in soil-water and in soil-water-surfactant systems. The kinetic pattern of desorption was biphasic, a slow desorption following an initial fast phase. Both phases followed first-order kinetics. The desorption rate of the first phase (K1) was very low in water for both herbicides and always increased in the presence of surfactants. At zero time, K1 increased 9- and 8-fold (atrazine), and 24- and 17-fold (linuron) in the presence of the two surfactants, respectively. Desorption rates decreased with the increase in the aging time in all three desorption systems. After 9 months of soil-herbicide aging time, DT25 for linuron was 6.85 h (SDS) and 41.7 h (Triton X100) and for the atrazine it was only possible determine in SDS solution (17.2h). The amount of desorbed herbicide in the different systems varied from 35.6–12.5% (water), 87.9– 46.2% (SDS), and 63.2–18.0% (Triton X-100) for atrazine and 8.02–3.94% (water), 69.9–41.3% (SDS), and 58.1–34.8% (Triton X-100) for linuron. The ratio of amount desorbed in surfactant solution and in water for the different aging times of the herbicides was greater for the desorption of linuron than that of atrazine. For both herbicides, it was always greater with SDS than with Triton X-100, and was higher when desorption of the residues aged for 9 months was carried out. The results indicate the interest of surfactants for increasing the desorption of atrazine and linuron from soils polluted with these compounds after a long aging time in the soil. Therefore, they indicate the possibility to use the pump-and-treat remediation technique for pesticides in soils with a long history of pollution. The enhanced desorption achieved will be governed by the hvdrophobic character of the herbicide, the nature of the surfactant used, the aging time, and the characteristics of soils.
In recent year, the development of chemical, physical, or physico-chemical techniques for the remediation of soils and
*Correspondence to: M. J. Sanchez-Martin; email:
[email protected]
water polluted by toxic non-ionic organic compounds has attracted increasing interest (Kearney and Roberts 1998; Smith and Burns 2001). One of the most widely used techniques is pump-and-treat, which involves in situ washing of the soil and later extraction of the water through wells excavated for treatment at the surface. This system may be efficient in the case of compounds that are soluble in the aqueous phase. However, its efficiency is very low for sparingly soluble and hydrophobic compounds, especially when the sources of pollution are soils with high organic matter (OM) contents. Different authors have shown, and it is generally accepted, that this parameter is very important in the adsorption-desorption process of hydrophobic compounds by soils and the adsorption of such compounds can be considered as a partition of the solute in the soil organic matter (Chiou 1989; Kile et al. 1995). Although adsorption phenomena are reversible, in practice adsorbed hydrophobic compounds are increasingly difficult to desorb as they become gradually sequestered in inaccessible microsites within the soil matrix (Alexander 1995, Pignatello and Xing 1996, Weber and Huang 1996). An increase in the efficiency of the pump-and-treat system has been achieved using surfactants, especially for the extraction of compounds that desorb slowly from the soil to the water. Surfactants are amphiphilic molecules having two major components (moieties), a hydrophilic or water-soluble moiety (head group) and a hydrophobic, or water-insoluble, moiety (tail group). At low concentrations, surfactants exist solely as monomers. As from a specific concentration, known as the critical micellar concentration (cmc), surfactants form self-aggregates (micelles) (Rosen 1989). At concentrations above the cmc, surfactants may strongly increase the solubility of organic pollutants by pollutant partition at the hydrophobic core of the surfactant micelles (Valsaraj et al. 1988; West and Harwell 1992; Di Cesare and Smith 1994). Over the past few years, several authors have studied the surfactant-enhanced desorption of organic pollutants and organic pesticides adsorbed in the soil. Thus, studies have been conducted to determine the effect of the chemical structure of the surfactant, its hydrophilic-lipophilic character, or its concentration in the soil-water system in this process (Deitsch and Smith 1995; Doong et al. 1996; Rouse et al. 1996;
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Deshpande et al. 2000; SEnchez-Camazano et al. 2003). Some authors have also studied the influence of soil characteristics and of desorbed organic compounds (IglesiasJimFnez et al. 1996, SEnchez-Martn et al. 2003, RodrguezCruz et al. 2004) and the interaction between the surfactant and the soil (Cano and Dorn 1996, Salloum et al. 2000, Zhu et al. 2003), factors that strongly affect the efficiency of surfactant-enhanced desorption. Nevertheless, few works reported in the literature have addressed the influence of the time of residence in the soil (aging time) of organic compounds in their desorption in soil-water-surfactant systems (Yeom et al. 1996; Paterson et al. 1999). In experiments on the surfactant-enhanced desorption of hydrophobic compounds conducted in laboratories, desorption is normally accomplished immediately after the adsorption process. Such studies are thus limited by the fact that the pollutant is only in contact with the soil for a short time period. As the residence time of hydrophobic organic residues in the soil increases, these undergo a series of processes, such as redistribution of the compound from weak adsorption sites to stronger ones, chemisorption or formation of covalent bonds between the compounds and the soil organic matter (Pignatello and Xing 1996; White et al. 1997), or oxidative coupling reaction during OM aging (Senesi 1993; Bollag et al. 1998). Such processes lead to a situation in which the organic compound may be strongly adsorbed by the OM and may remain strongly retained in the soil (Gevao et al. 2000; Barraclough et al. 2005). As a result, their desorption is more difficult, and both the desorption rate and the amount of compound desorbed decrease. These observations have important implications for the application of remediation technologies to polluted soils since such technologies are generally used in the extraction of these compounds from soils with a long contamination history. In light of the above, we though it is appropriate to study the influence of the aging time on the desorption of the herbicides atrazine and linuron adsorbed in a soil with a high OM content (7.3%) in the presence of the surfactant Triton X-100 (non-ionic) and SDS (anionic). The aim of this work was to know the efficiency of the surfactants in the desorption of aged residues of organic pesticides. To accomplish this, we studied the desorption rate (desorption kinetics) and the mass transfer (desorption isotherms) of the herbicides adsorbed in the soil after 0, 3, and 9 months of incubation in soil-water and in soil-water-surfactant systems. Atrazine and linuron are two herbicides of different hydrophobic character. Both are adsorbed by the organic matter of the soil (Garca-ValcErcel et al. 1998; SEnchezCamazano et al. 2000) and both have been detected in surface and ground waters in different countries (Jayachandran et al. 1994, Eke 1994, Spalding et al. 2003). Bound residues of these compounds have also been detected in different soils (Walker 1976; Barriuso et al. 1991; Topp et al. 1994; Johnson et al. 1999; Rodrguez-Cruz et al. 2001). SDS and Triton X-100 are considered to be effective surfactants for the removal of hydrophobic pollutants from different supports (Deitsch and Smith 1995; SEnchez-Camazano et al. 2003) and are considered to be biodegradable surfactants (Swisher 1987; Holt et al. 1992).
Materials and Methods Soils A sandy loam soil with an organic matter content of 7.28% was selected for the study. This soil contained 61.8% sand, 18.9% silt, and 19.3% clay, having illite and kaolinite in its clay fraction, Soil pH was 5.2. Soil samples were air-dried and sieved through 2-mm mesh. Their particle size distributions were determined using the pipette method. Organic carbon was determined according to a modified version of the method of Walkley-Black (Jackson 1958), the result being multiplied by 1.72 for conversion into OM contents. Soil pH values were measured in slurries made up at a 1:1 soil/water ratio. Clay minerals were identified by the X-ray diffraction technique.
Herbicides Atrazine (2-chloro-4-(ethylamino)-6-isopropylamino-1,3,5-triazine) and linuron (N¢-(3,4-dichlorophenyl)-N-methoxy-N-methylurea) were used in the study. Both are moderately hydrophobic compounds: atrazine has a water solubility of 33 mg L)1 and a log Kow of 2.5, and linuron has a solubility of 81 mg L)1 and a log Kow of 3.0 (Tomlin 2000). Because of its solubility in water, atrazine should be more hydrophobic than linuron. However, in view of its Kow value linuron must have greater affinity for hydrophobic organic compounds and materials than atrazine. 14C-labelled atrazine (97% pure) with a specific activity of 4.13 MBq mg)1 and 14C-labelled linuron (98% pure) with a specific activity of 1.31 MBq mg)1 were supplied by International Isotope (MKnchen, Germany). Unlabelled, analytical grade atrazine (98% pure) and linuron (99% pure) were supplied by Labor Dr. Ehrenstorfer (Augsburg, Germany). Atrazine metabolites (98% pure) 2-hydroxyatrazine (2-hydroxy-4-(ethylamino)-6-isopropylamino-1,3,5-triazine), de-ethylatrazine (2-chloro-4-amino-6-isopropylamino-1,3,5-triazine) and de-isopropylatrazine (2-chloro-4(ethylamino)-6-amino-1,3,5-triazine) were supplied by Labor Dr. Ehrenstorfer (Augsburg, Germany). Linuron metabolites (99.5% pure) N-(3,4-dichlorophenyl)-N¢-methylurea, N-(3,4-dichlorophenyl)-N¢methoxyurea, N-(3,4-dichlorophenyl)urea and 3,4-dichloroaniline were supplied by HoMchst AG (Germany).
Surfactants The anionic surfactant sodium dodecyl sulphate (SDS) and the nonionic surfactant t-octylphenoxypolyethoxyethanol (Triton X-100) were selected for the study and were supplied by Sigma Adrich Chemical Co. (Milwaukee, WI). The concentrations of surfactant used in the desorption and kinetic studies were 10- and 100-fold the critical micelle concentration (cmc) for SDS and Triton X-100, respectively. The cmc of SDS is 2.38 g L)1 and of Triton X-100 is 0.15 g L)1.
Adsorption Isotherms Atrazine and linuron adsorption isotherms by the soils in aqueous medium were obtained by treating 5 g of soil (<2 mm) with 10 mL of an aqueous solution of 14 C-labelled herbicide at concentrations of 5, 10, 15, 20, and 25 (lg mL)1 and an activity of 200 Bq mL)1 in glass tubes of 25 mL. HgCl2 was added to the aqueous solutions at a concentration of 1.84 mmol kg)1 to eliminate microbiological activity under aerobic conditions, thus preventing biodegradation of herbicides (Yeom et al. 1996). The suspensions were shaken for 24 h
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at 20 € 2C in a thermostatted chamber, with intermittent shaking (2 h every 3 h). Preliminary experiments revealed that contact for 24 h was long enough for equilibrium to be reached. Following this, the suspensions were centrifuged at 5045g for 30 min, after which the concentration of atrazine or linuron was determined in the equilibrium solution. All experiments were carried out in duplicate. The dpm value recorded for the supernatant aliquots was related to the dpm obtained for the aliquots of the respective standards of herbicide solutions, and the equilibrium concentration of each compound was determined. The amount of pesticide adsorbed was considered to be the difference between that initially present in solution and that remaining after equilibrium with the soil.
Aging Experiments Samples of soil were treated with 25 lg mL)1 (the highest concentration used for the isotherms) and after the adsorption they were prepared to carry out desorption experiments. All possible volume of the equilibrium solution was retired from the glass tubes; the remaining volume was determined by weighing, and the solution concentration in the residual solution was assumed to be the same as that measured in the bulk supernatant. Under these moisture conditions (close to the water-holding capacity) the tubes were sealed and were incubated in a thermostatted chamber at 20 € 2C for 3 and 9 months. Soil samples with adsorbed atrazine and linuron used as reference (not incubated) were desorbed immediately after their preparation. The possible degradation of atrazine and linuron during incubation was controlled in samples prepared in a similar way, using the same method but with non-labelled herbicide solutions.
Oxidizer (RJ. Harvey X-500 Instrument Corporation) under conditions of excess of O2 at 900C. Samples were combusted for 4 min. The 14CO2 was trapped in 10 mL of scintillation cocktail (Harvey) and 1 mL of ethanolamine, after which determinations were made by liquid scintillation counting in a Beckman LS6500 scintillation counter.
Determination of Labelled and Non-Labelled Atrazine and Linuron Quantitative determination of the two 14C-labelled herbicides was accomplished on a Beckman LS6500 liquid scintillation counter. The activity of the solutions, measured in disintegrations per minute (dpm), was determined in 1 mL of supernatant to which 4 mL of scintillation cocktail had been added. Determinations were carried out in duplicate for each supernatant. Quantitative determination of non-labelled herbicides and their metabolites was accomplished by HPLC. The apparatus used was a Waters chromatograph (Waters Assoc., Milford MA), equipped with a model 600E multisolvent delivery system attached to a model 717 plus autosampler, a model 996 photodiode array detector (DAD), and as Millennium 2010 chromatography manager data acquisition and processing system. A Novapak C18 (150 mm · 3.9 mm I.D., Waters Assoc.) column was used at ambient temperature and the mobile phase was 65:35 methanol/water. The flow rate of the mobile phase was 1.0 mL min)1 and the sample injection volume was 10 lL.
Data Analysis Kinetic Studies Batch kinetic studies were conducted in aqueous medium and in surfactant medium to determine the desorption rate of herbicides from soil-herbicide samples incubated for 0, 3, and 9 months. After each of these incubation periods, samples were treated with a volume of water or surfactant solution similar to the water volume previously withdrawn after adsorption. The suspensions were shaken for 5 min, 1 h, 4 h, 8 h, 14 h, 24 h, 36 h, and 48 h at 20 € 2C in a thermostatted chamber, after which they were centrifuged at 5045g for 30 min. Following this, the concentration of atrazine or linuron was determined in the equilibrium solution.
Freundlich equation was used to describe the results of adsorption (log Cs = logKf + nf logCe) and desorption (log Cs = logKfd + nfd logCe) of the herbicides by the soil in the different systems, as is usual in this kind of study (Ma and Selim 1996, Moreau-KervFsan and Mouvet 1998). The desorption rate was described by a first-order kinetic equation log (Cs-C) = )Kt/2.3 + log Cs, where Cs is the amount initially adsorbed, C is the amount desorbed at different times, and K is the rate constant. Using this equation, we determined the rate constants for each desorption phase and the DT25 and DT50 parameters relative to the time required for desorption of 25% and 50% of the compounds.
Desorption Isotherms
Results and Discussion
Atrazine and linuron desorption isotherms were obtained in duplicate from soil-herbicide samples incubated for 0, 3, and 9 months after adsorption. The volume withdrawn after adsorption was replenished by weighing with the same volume of water or surfactant solution. These suspensions were shaken for an equilibrium time of 24 h (as determined from the kinetic experiments) at 20 € 2C, and then centrifuged, following the same method as that used to obtain the adsorption isotherms. Desorption was carried out in four consecutive steps. Each desorption step was accomplished by replacing half of the supernatant volume by water or surfactant solution of the concentration studied. The amount of herbicide desorbed from the solid to solution at each desorption step was calculated based on the change of 14 C-herbicide concentration in the solution phase. After the fourth desorption step, a mass balance was performed for each herbicide. Total 14C-activity present in the soil samples was determined by combustion of triplicate 1-g dried soil samples, using a Biological
Kinetic Study of Herbicide Desorption in the Soil-Water and Soil-Water-Surfactant Systems Figures 1 and 2 show the curves corresponding to atrazine and linuron desorption as a function of time from herbicide-soil samples after incubation for 0, 3, and 9 months in aqueous medium and in SDS solution of 10 cmc and Triton X-100 solution of 100 cmc. The logarithmic plot of the adsorbed amounts remaining in the soil after desorption indicates a biphasic pattern of desorption of both herbicides (Figs. 3 and 4). Each of these phases can be fitted to a straight line, showing that the desorption of atrazine and linuron follows first-order kinetics in both phases, although with different rates. Desorption in the first phase is fast, while in the second phase
Desorption of Aged Atrazine and Linuron in Soil
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Fig. 1. Atrazine desorption as a function of time from herbicide-soil samples in water and in SDS and Triton X-100 solutions after zero months (A), three months (B), and nine months (C) of aging time
Fig. 2. Linuron desorption as a function of time from herbicide-soil samples in water and in SDS and Triton X-100 solutions after 0 months (A), 3 months (B), and 9 months (C) of aging time
it occurs slowly, both in water and in the presence of the surfactants. In general, equilibrium is reached from 24 h although in some systems a slow desorption of the compounds continues for longer time. This type of behaviour has frequently been reported by other authors for the desorption in water of organic compounds and pesticides from soils (Kookana et al. 1992; Pavlostathis and Mathavan 1992). In the presence of different surfactants, Paterson et al. (1999) reported a period of 50 h of treatment for the stationary phase to be reached in the desorption of phenanthrene and anthracene, and Yeom et al. (1996) observed a similar equilibrium period for low surfactant concentrations, although longer equilibrium times when concentrations of surfactant greater than 3% were used.
K1 and K2 rate constants for the desorption of herbicides in each of the phases and in the different systems (water, water-SDS, and water-Triton X-100) were calculated. K1 values are shown in Table 1 for atrazine and linuron, K2 values (not included) were, in general, two orders of magnitude lower than K1, and they were negative for desorption in water because there was practically no variation in the amounts desorbed as a function of time in second phase (Johnson et al. 1999). These observations indicate that the desorption of the herbicides is governed by the rate of the first phase. Table 1 also includes the DT25 values, relative to the desorption of 25% of the compound. DT50 values corresponding to the desorption of 50% of the compounds are not included, even though this is usual practice in kinetic
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Fig. 3. Desorption kinetics of atrazine in soil-water and in soil-watersurfactant systems after 0 months (n), 3 months (m), and 9 months (·) of aging time
studies, since this desorbed amount was only reached in the water-SDS system. The K1 rate constants for atrazine desorption decreased with the increase in the aging time of the herbicide in the soil for all three desorption systems. When desorption was carried out immediately after adsorption (t = 0), the highest K1 constant was observed for SDS (0.120 h)1). This constant was similar to that found for Triton X-100 (0.107 h)1), and both constants were almost 10-fold higher than the desorption K1 in water (0.013 h)1). Upon increasing the time of contact of the herbicide with the soil, the K1 constants decreased by one order of magnitude in all three systems although these constants remained higher in the water-surfactant system than in the water system. Thus, after an aging time of the herbicide in soil of 3 months, K1 constants in SDS and in Triton X-100 were 7- and 3-fold
M. S. Rodriguez-Cruz et al.
Fig. 4. Desorption kinetics of linuron in soil-water and in soil-watersurfactant systems after 0 months (n), 3 months (m), and 9 months (·) of aging time
higher, respectively, than K1 in water, and after an aging time of 9 months, K1 constants in SDS and in Triton X-100 were 3- and 2-fold higher, respectively, than K1 in water. The DT25 values determined from the desorption rate constants of the first stage were 21.4 h, 2.40 h, and 2.68 h for the atrazine desorption immediately after adsorption (t = 0) in water and in the SDS and Triton X-100 solutions, respectively. After 3 and 9 months of sample incubation, it was not possible to calculate DT25 in water since 25% of desorption was not reached in the first phase of the kinetic. In the presence of the surfactants, DT25 increased with respect to the values obtained at zero time but the effect was very different for both surfactants. In SDS solution, DT25 increased 3–7-fold after 3 and 9 months of sample incubation, respectively, whereas in Triton X-100 solution DT25 was only attained after 3 months of aging time and it was not possible to achieve this desorption after 9 months of sample incubation (Table 1).
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Desorption of Aged Atrazine and Linuron in Soil
Table 1. Desorption rate constants (K1) for atrazine and linuron in water and in solutions of SDS and Triton X-100 after different aging times in soil and DT25 values Water Aging time Atrazine 0 months 3 months 9 months Linuron 0 months 3 months 9 months
SDS
Triton X-100
K1 € SD (h)1)
DT25
K1 € SD (h)1)
0.013 € 0.004 0.007 € 0.001 0.005 € 0.002
21.4 — —
0.120 € 0.008 0.046 € 0.005 0.017 € 0.006
2.40 6.22 17.2
0.107 € 0.032 0.020 € 0.003 0.012 € 0.003
2.68 14.2 —
0.019 € 0.004 0.002 € 0.001 0.002 € 0.001
— — —
0.452 € 0.012 0.049 € 0.007 0.042 € 0.006
0.63 5.76 6.85
0.317 € 0.018 0.033 € 0.010 0.028 € 0.008
0.91 8.64 41.7
DT25
K1 € SD (h)1)
DT25
K1 € SD, mean values € standard deviation of two duplicates. —, not determined.
Upon increasing the aging time of the herbicide in the soil, K1 rate constants for linuron also decreased by one order of magnitude in the three desorption systems studied. The K1 constant in water was very low when desorption was carried out immediately after adsorption (t = 0); under these conditions, K1 increased 24- and 17-fold in the presence of SDS and Triton X-100, respectively. When the time of sample incubation increased, this increase in K1 remained constant (desorption in SDS) or decreased slightly (desorption in Triton X-100). The desorption rate of linuron in water was lower than that of atrazine after similar aging time of herbicide in the soil. Despite this, the desorption rate of linuron in the water-surfactant system was higher. The time required for the desorption of 25% of the linuron in water was not possible to determine for any of the sample incubation times assayed (Table 1). Rodrguez-Cruz et al. (2001) have reported high persistence for linuron in soils amended with different organic materials (DT50 > 9 months). However, DT25 was less than 1 h when desorption was performed immediately after adsorption (t = 0) in the presence of surfactants. The DT25 increased 9- and 11-fold in the presence of SDS and 10- and 46-fold in the presence of Triton X-100 when the aging time of the residues was increased but the desorption of 25% of linuron in the water-surfactant system always occurred during the first stage of desorption.
Desorption Study of Herbicides in the Soil-Water and SoilWater-Surfactant Systems
Fig. 5. Desorption isotherms of atrazine from soils in water system, and water-surfactant system after 0 months (n), 3 months (m), and 9 months (·) of aging time
Figures 5 and 6 show the desorption isotherms of atrazine and linuron obtained in water and SDS and Triton X-100 solutions from soil-herbicide samples incubated for 0, 3, and 9 months, together with the adsorption isotherms of the herbicides by the soil. Adsorption isotherms fit the Freundlich equation, with values of r ‡ 0.99, the Kf and nf values obtained from this equation being 5.52 and 0.82 for atrazine and 20.3 and 0.81 for linuron. The desorption isotherms of atrazine also fit this equation with r values between 0.88 and 0.97, with the exception of those obtained in surfactant medium at zero time (no aging). These isotherms point to a fairly strong desorption of the herbicide in the first washing with the surfactant. The values of the desorption constants, Kfd and nfd, are shown in Table 2.
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Fig. 6. Desorption isotherms of linuron from soils in water system, and water-surfactant system after 0 months (n), 3 months (m), and 9 months (·) of aging time
Kfd values (representing the amount of herbicide remaining adsorbed after desorption for an equilibrium concentration equal to 1) increased and nfd values decreased in all the systems studied with the increase in the aging time of the herbicide. In the presence of surfactants, Kfd values were lower than in water after 3 and 9 months of aging time. Although we failed to find any data relating to the influence of surfactants in the desorption of aged pesticide residues, the literature does offer data concerning the enhanced desorption of pesticides in the presence of dissolved organic carbon (DOC). The action mode of DOC, according to the micellar model proposed by Wershaw (1986), can be considered similar to that of surfactants. In this sense, Barriuso et al. (1992) have reported that an apparent enhanced solubility of atrazine results from its
M. S. Rodriguez-Cruz et al.
adsorption to DOC and Gao et al. (1998) have also reported an enhanced desorption of adsorbed atrazine by sediments in the presence of DOC. The desorption isotherms show hysteresis resulting from the discrepancy between the adsorption and desorption isotherms. Hysteretic behaviour for atrazine desorption in water has been reported by different authors (Ma et al. 1993; Laird et al. 1994; Moreau-KervFsan and Mouvet 1998; Lesan and Bhandari 2003). The values of the hysteresis coefficients, H (Table 2), determined from the nf/nfd ratio (Barriuso et al. 1994), were higher in the samples incubated, indicating an increase in the irreversibility of adsorption with time, also reported by others (Ma et al. 1993). In surfactant solution, the desorption isotherms show less hysteresis than in water; the H coefficients for the desorption isotherms obtained in SDS were 5–6-fold lower than those obtained in water. According to these coefficients, the presence of the surfactants SDS and Triton X-100 at concentrations higher than the cmc would decrease the irreversibility of atrazine adsorption. Desorption of the herbicide from the soil to the water-surfactant system will be enhanced even though the aging time of the residues is high. Table 2 shows the enhanced solubility of atrazine in SDS and Triton X-100 solutions after four successive washings, expressed as the percentage of herbicide desorbed in relation to the amount initially adsorbed (D) and by the efficiency coefficient (E) defined as the relationship between the percentage of atrazine desorbed in surfactant solution and the percentage of this herbicide desorbed in water at different aging times. Atrazine desorption in water was 35.6% when desorption was performed immediately after adsorption, and it decreased to 12.5% after 9 months of aging of the herbicide. The percentages of desorption increased in the presence of the surfactants, although they also decreased with the increase in the aging time of the herbicide in the soil. The efficiency coefficients varied from 2.47 (zero time) to 3.70 (9 months of incubation) in presence of SDS and from 1.78 to 1.44 in presence of Triton X-100 during the same incubation times. The efficiency of SDS in atrazine desorption was always greater than that of Triton X-100 and increased with the increase in the aging time of the herbicide. These differences could be explained in terms of the adsorption of both herbicides by the soil. In a previous work, we observed higher distribution coefficients for the adsorption of Triton X100 by soils than for the adsorption of SDS (SEnchezCamazano et al. 2003). This increase in adsorption will give rise to a decrease in the concentration of surfactant in solution, which could contribute to a decrease in its efficiency even though the concentrations used here were much higher than the cmc of the surfactant. Atrazine residues in soil were determined after desorption of the herbicide in the systems studied for each of the aging periods by 14C-combustion. The 14C total balance was greater than 85%; the residual amount increased with the aging time of the herbicides in the soil. The amounts of 14C-atrazine after 9 months of aging time were 72% (water), 40% (SDS) and 68% (Triton X-100). They indicated that herbicide desorption is difficult even in the presence of surfactants. Atrazine residues bound to soil OM have been noted by different authors (Barriuso et al. 1991, Loiseau and Barriuso 2002). Additionally, control of the possible degradation of atrazine in each of the incubation periods indicated the presence of traces of the
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Desorption of Aged Atrazine and Linuron in Soil
Table 2. Freundlich desorption constants (Kfd, nfd) and hysteresis coefficients (H) for atrazine desorption in water and in SDS and Triton X-100 solutions at different aging times. Amount of atrazine desorbed (D) and efficiency coefficients (E) Desorption 0 months aging time Water SDS Triton X-100 3 months aging time Water SDS Triton X-l00 9 months aging time Water SDS Triton X-100
Kfd € SD
nfd € SD
H € SD
D € SD %
E
18.7 € 0.43 — —
0.30 € 0.00 — —
2.89 € 0.02 — —
35.6 € 0.60 87.9 € 0.05 63.2 € 0.35
2.47 1.78
25.6 € 1.48 9.34 € 0.24 20.2 € 1.37
0.10 € 0.02 0.45 € 0.01 0.20 € 0.02
8.70 € 1.24 1.93 € 0.04 4.37 € 0.33
21.3 € 2.37 56.7 € 0.63 32.8 € 2.15
2.66 1.54
27.8 € 0.66 13.6 € 0.14 26.9 € 0.93
0.05 € 0.01 0.31 € 0.00 0.07 € 0.01
16.1 € 2.63 2.78 € 0.01 12.0 € 0.95
12.5 € 0.67 46.2 € 0.36 18.0 € 1.53
3.70 1.44
Mean values € Standard deviation of two duplicates. —, Not determined owing to high desorption after the first washing with the surfactant.
Table 3. Freundlich desorption coefficients (Kfd, nd) and hysteresis coefficients (H) for linuron desorption in water and in SDS and Triton X-100 solutions at different aging times. Amount of linuron desorbed (D) and efficiency coefficients (E) Desorption 0 months aging time Water SDS Triton X-100 3 months aging time Water SDS Triton X-100 9 months aging time Water SDS Triton X-100
Kfd € SD
nfd € SD
H € SD
D € SD %
E
43.4 € 0.41 — —
0.09 € 0.00 — —
8.80 € 0.20 — —
8.02 € 0.36 69.9 € 2.69 58.1 € 0.53
8.72 7.24
43.8 € 2.24 — —
0.06 € 0.02 — —
13.5 € 5.21 — —
6.68 € 2.05 56.1 € 1.72 48.8 € 0.38
8.40 7.31
44.4 € 0.10 — —
0.02 € 0.00 — —
40.5 € 3.87 — —
3.94 € 0.32 41.3 € 7.54 34.8 € 6.56
10.5 8.83
Mean values € Standard deviation of two duplicates. —, Not determined owing to high desorption after the first washing with the surfactant.
metabolite hydroxiatrazine in solution, possibly arising from chemical degradation in the three systems studied. The hydrolysis of atrazine to hydroxiatrazine is considered to be a chemical process catalysed by the soil surface and favoured by high OM contents (Ma et al. 1996). The desorption isotherms of linuron (Fig. 6) indicate that only those obtained in water fit the Freundlich equation, with values of r ‡ 0.72. Desorption constants Kfd and nfd (Table 3) were slightly modified upon altering the aging time of the linuron residues in the soil. The hysteresis coefficient H increased from 8.80 to 40.5, indicating an increase in the irreversibility of adsorption upon increasing the aging time. Hysteretic behaviour for linuron desorption was more striking than that of atrazine, possibly due to its more hydrophobic character as indicated by the Kow value. Spurlock and Biggar (1994) reported that the sorption process of phyenylurea compounds such as linuron, which contains polar groups in its molecule, is more complex than hydrophobic theory suggests and specific interactions must occur between these herbicides and the OM and mineral fractions. The percentages of linuron desorbed (D) were very low in water and decreased from 8.02% to 3.94% after 9 months of incubation of the residues in soil. The desorbed amounts were higher in the presence of surfactants ranging from
69.9% to 41.3% in SDS and from 58.1% to 34.8% in Triton X-100. The efficiency coefficients in the desorption of linuron after 0, 3, and 9 months of sample incubation were slightly higher in the presence of SDS (8.72–10.5) than in the presence of Triton X-100 (7.24–8.83). These coefficients were higher after 9 months of aging time indicating that the efficiency of SDS and of Triton X-100 in the desorption of linuron increased with the increase in the time of contact between the residues and the soil. Also, the efficiency of both surfactants was higher for the desorption of linuron than for that of atrazine, possibly due to its more hydrophobic nature (Valsaraj et al. 1988; Doong et al. 1996; SEnchez-Camazano et al. 2003). The 14C total balance in the samples after desorption in water or surfactants for each of the aging times considered indicated 14C total percentages higher than 87%. The 14C retained in the soil after the 9 month aging time was nearly two-fold lower after extraction in SDS (44%) or Triton X-100 (50%) than in water (82%). The analytical study carried out in parallel to determine the presence of possible metabolites of linuron in the soil indicated the presence of small amounts of metabolites N-(3,4-dichlorophenyl)-N¢-methylurea, and N-(3,4-dichlorophenyl)urea only after 9 months of incubation when desorption was conducted with SDS. These metabolites
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are due to the biological degradation that may occur after nine months of contact even though the conditions under which incubation was performed were sterile (Maier-Bode and Hartel 1981).
Conclusions The results obtained show the low efficiency of water for the desorption of atrazine and linuron from aged residues of both herbicides. The efficiency was lower for linuron than for atrazine. They also show the possibility of achieving an increase in efficiency in the desorption of residues of both herbicides after different aging times in the soil in solutions of the surfactants SDS (anionic) and Triton X-100 (non-ionic). The efficiency of the desorption of both pesticides was always greater in the water-SDS system than in the water-Triton X100 system. Unlike what happens in water, the increase in the desorption of the herbicides in the water-surfactant system was greater for linuron than for atrazine, possibly owing to its more hydrophobic nature. The most striking increase was seen in the desorption of linuron from residues aged for 9 months in the presence of both surfactants, the amount of herbicide desorbed being 10-fold (SDS) and 9-fold (Triton X-100) higher than that desorbed in water. Accordingly, the results offer information suggesting that it might be possible to extend use of the pumpand-treat technique to the recovery of soils with a long history of contamination.
Acknowledgments. This work was financially supported by the Spanish ‘‘Comisio´n Interministerial de Ciencia y Tecnologı´ a’’ as a part of Project AMB97-0334.
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