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Instituto de Recursos Naturales y Agrobiología de Salamanca, CSIC, Apdo 257, 37071 Salamanca, Spain
* Corresponding author (mjesussm{at}usal.es).
Received for publication August 13, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: cmc, critical micelle concentration • D, amount of herbicide desorbed • E, efficiency coefficient • H, hysteresis coefficient • OM, organic matter
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Pump-and-treat systems are effective only for contaminants that are in the aqueous phase. However, in the case of hydrophobic pollutants that are only sparingly soluble this system is slow and inefficient. An increase in the efficiency of the process has been achieved with in situ surfactant-enhanced soil flushing technology, mainly applied for non-aqueous phase liquids (NAPLs), which are the most common pollutants of soil and ground water (Kimball, 1994).
Surfactants are good solvents of sparingly soluble organic compounds (Valsaraj et al., 1988; Edwards et al., 1991; Pennell et al., 1997) and they increase NAPL mobility, allowing soil remediation by the pump-and-treat system. The surfactant polar head group interacts strongly with the water phase, and the hydrocarbon chain portion partitions into the organic contaminant phase, reducing the interfacial tension between phases (Rosen, 1989; West and Harwell, 1992; Di Cesare and Smith, 1994). Because surfactant molecules can exist in several forms (micelles, monomers dissolved in liquid or adsorbed at an interface), they can enhance contaminant mobility in several ways. Among these are increasing the solubility of a contaminant; reducing the surface tension of trapped NAPL residual globules, rendering them flushable; forming micelles that can be flushed out with pumping; and desorbing the contaminant from soil particles (Kimball, 1994).
Of all the above processes, the study of surfactant-enhanced desorption for organic pollutants adsorbed on soils has been addressed by many investigators in recent years (Fountain et al., 1991; Sun et al., 1995; Rouse et al., 1996; Paterson et al., 1999; Deshpande et al., 1999, 2000) although such information can only be considered a beneficial side effect in the context of major engineered remediation processes.
In the study of surfactant-enhanced desorption it is necessary to take into account the characteristics of the surfactant (e.g., chemical structure, hydrophilic–lipophilic balance [HLB], or cmc), its concentration in the soil–water system, the solubility and hydrophobicity of the contaminant, and the characteristics of soil (e.g., OM, clay content). Enhanced solubility of pollutants has been clearly indicated by several authors at surfactant concentrations higher than the cmc. However, at surfactant concentrations below the cmc competitive adsorption of an organic compound by soil and/or by a surfactant in solution may occur, and hence an increase or decrease in desorption of compound from soil, depending on the characteristics of soil and organic compound (Di Cesare and Smith, 1994).
Another important aspect to consider is the interaction of surfactant with soil, since it may, on one hand, alter the surfactant concentration in solution, thereby decreasing its efficiency for desorption, and on the other, alter the soil surface, where surfactant molecules may be adsorbed in the form of monomer or forming hemicelles or admicelles (West and Harwell, 1992). This surfactant adsorption increases the organic C content of soil and the extent of its hydrophobic surfaces, which may contribute to a decrease in the organic compound desorption.
The work performed along these lines has mainly focused on the study of aromatic or chlorinated hydrocarbons, which are very hydrophobic compounds. However, among the compounds studied pesticides have seldom been addressed even though the presence of pesticides in soil and water resources is increasingly more frequent (Eke, 1994; Kookana et al., 1998; Tanabe et al., 2001; Spalding et al., 2003) and of growing concern.
Most pesticides used in agriculture are moderately hydrophobic compounds, with a wide range of complex molecular structures that differ from hydrocarbons in their lower hydrophobicity and in the presence of polar functional groups. These compounds are also strongly adsorbed by soil OM and their desorption rate from the soil phase to water is limited (Khan, 1982; Cheng, 1990; Barriuso and Koskinen, 1996). Thus, surfactants could be efficient in favoring pesticide desorption from soils. The literature contains some references concerning the effect of surfactants on the behavior of pesticides in the soil, especially with regard to modifications in their mobility (Sánchez-Camazano et al., 1995a, 1997; Singh and Kumar, 2000; Prima et al., 2002) or adsorption (Iglesias-Jiménez et al., 1997; Abu-Zreig et al., 1999; Beigel and Barriuso, 2000; Dai et al., 2001; Locke et al., 2002).
The use of surfactants to increase the desorption of pesticides from soil as a potential means of remediation of soils and waters polluted by pesticides requires that several factors must be analyzed. These include the efficiency of surfactants in enhancing desorption, the economic cost, and the environmental implications of degradation products resulting from the biodegradation of surfactants (Swisher, 1987; Holt et al., 1992).
The authors of the present work previously performed a study (Sánchez-Camazano et al., 2003) on the efficiency in the desorption of atrazine and linuron from soils for a series of selected surfactants of different nature (three anionic and eight non-ionic) and with different cmc or hydrophilic–lipophilic balance. From the results obtained, it was possible to establish interesting relationships between surfactant efficiency and their concentration, indicating (i) an increase in the efficiency of surfactants with the absolute cmc value, regardless of surfactant structure, and (ii) an increase in the efficiency of a particular surfactant with the surfactant absolute concentration in solution, although for similar absolute concentrations of different surfactants a greater efficiency was seen in the system with highest micellar concentration (i.e., with the decrease in the values of surfactant cmc).
With a view to gaining further insight into soil characteristics in surfactant-enhanced desorption of atrazine and linuron from soils, an issue that was not taken up in that previous work, here we performed more detailed studies to cover this aspect, which required a higher number of different soils. Based on our previous results, we selected two surfactants: one anionic, sodium dodecylsulfate (SDS) (Sánchez-Camazano et al., 2000b; Sánchez-Martín et al., 2003), and the other non-ionic, Triton X-100, using five natural soils with different OM and clay contents.
The present study is conducted with the non-ionic surfactant Triton X-100 and its aim is to contribute to a better understanding of different molecule and surface interactions in the complex soil–herbicide–water–surfactant system. We discuss the competitive adsorption of herbicides by soil and surfactant in solution, the enhanced solubility of herbicides in surfactant solution, and the influence upon these processes by the adsorption of surfactant by soils. The herbicides atrazine and linuron were chosen based on their different hydrophobic character and the presence of different functional groups in their molecule. Additionally, atrazine has been found in numerous monitoring studies of ground and surface water for different countries (Sánchez-Camazano et al., 1995b; Spalding et al., 2003) and is commonly used as a type herbicide for establishing protocols by various foreign legislatures. Currently, linuron has widespread use and is also often found in surface and ground waters (Eke, 1994; Carabias-Martínez et al., 2003).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Five natural (non-cultivated) soils from Salamanca province (northwestern Spain) were selected for this study (Table 1). Soil samples were air-dried and sieved through 2-mm mesh. Particle size distribution of clay (<2 µm), silt (2–50 µm), and sand (0.05–2 mm) content was determined for each soil using the pipette method (Day, 1965). The organic C content was determined by the Walkley–Black procedure (Jackson, 1958) with the results being multiplied by 1.72 for conversion into OM contents. Soil pH values were measured in slurries made in a 1:1 soil to water ratio. Clay minerals were identified by the X-ray diffraction technique (Robert, 1975).
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Adsorption Isotherms
Adsorption isotherms of 14C-atrazine and 14C-linuron by the soils were obtained using the batch equilibrium technique. Duplicate 5-g soil samples were equilibrated with 10 mL of an aqueous solution of each herbicide at concentrations of 5, 10, 15, 20, and 25 mg L–1 and an activity of 200 KBq L–1. The suspensions were shaken at 20 ± 2°C for 24 h in a thermostated chamber with intermittent shaking (2 h every 3 h). Preliminary experiments showed that contact for 24 h was long enough for equilibrium to be reached. Subsequently, the suspensions were centrifuged at 5045 x g for 15 min. To determine the pesticide concentration at equilibrium, a 1.0-mL aliquot of supernatant solution was mixed with 4 mL of scintillation liquid and its activity was measured in disintegrations per minute (dpm) on an LS 6500 liquid scintillation counter (Beckman Coulter, Fullerton, CA). The dpm value recorded for the supernatant aliquot 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 equilibration with the soil. Atrazine and linuron adsorption data were fitted to the linearized form of the Freundlich adsorption isotherm equation:
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Desorption Isotherms
Desorption isotherms of 14C-atrazine and of 14C-linuron were obtained from soil samples initially treated with 25 mg L–1 herbicide solution during the adsorption study. After the samples had been shaken and centrifuged, 5 mL of supernatant were removed from the adsorption equilibrium solution and immediately replaced by 5 mL of water or 5 mL of the Triton X-100 solutions at different concentrations. The resuspended samples were shaken for 24 h at 20 ± 2°C, after which the suspensions were centrifuged and the desorbed herbicide was measured as reported above. This desorption procedure was repeated four times for each soil. The amount of herbicide adsorbed by the soil at each desorption stage was calculated as the difference between the initial amount adsorbed and the amount desorbed. All determinations were performed in duplicate. To determine the global balance of the initial radioactivity, some soils randomly selected were oxidized after desorption of the herbicides using an OX-500 biological oxidizer (Harvey Instruments, Buffalo, NY). The global balance of radioactivity was always higher than 90%. Additionally, to check that the radioactivity detected in the soil corresponded to atrazine and linuron, randomly selected adsorption–desorption experiments were also performed using unlabeled pesticides. Determination of the pesticides was performed using high performance liquid chromatography with a Waters (Milford, MA) chromatograph following the methods of Sánchez-Camazano et al. (1995b)(2000a) to confirm the chemical identity of the herbicides. Herbicide desorption data were fitted to the linearized form of the Freundlich equation:
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Adsorption of Triton X-100 by Soils
Adsorption of Triton X-100 by the soils was also performed using the batch equilibrium technique. Duplicate 1-g soil samples were equilibrated with 10 mL of Triton X-100 solution at concentrations of 0.75, 1.50, and 50cmc. The suspensions were shaken for 24 h at 20 ± 2°C, after which they were centrifuged at 5045 x g for 30 min. Triton X-100 concentrations in the supernatants were determined by UV spectroscopy at 223 nm and/or 275 nm (Sun et al., 1995; Lee et al., 2000) using a Varian (Palo Alto, CA) Cary 100 spectrophotometer. Two linear ranges of Triton X-100 concentrations were used to determine the equilibrium concentrations: 10 to 80 mg L–1 (r 
0.99, p < 0.001) and 100 to 1000 mg L–1 (r 0.99, p < 0.001). All determinations were performed against a soil blank to correct for possible interferences in the measurement of the surfactant. The precision of the method was determined by carrying out the adsorption experiment 10 times for a soil sample at different concentrations (relative standard deviation was always <3%). The amount of surfactant adsorbed was calculated from the amount initially present in solution and that remaining after equilibrium with the soil. Distribution coefficients, Kd, as a measure of the adsorption of Triton X-100 by the soils, were obtained from the relationship between the amount of surfactant in the soil and in the equilibrium solution after adsorption.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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0.99. Adsorption coefficients (Kf and n), determined from Eq. [1], are shown in Table 2.
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The Kf values for linuron adsorption by the soils (Table 2) range between 4.76 and 33.3. The values of these affinity coefficients are higher than those obtained for atrazine adsorption by the same soils, but are of the same order as those reported by Sánchez-Camazano et al. (2000a) for linuron adsorption by natural soils (non-cultivated). The n values vary between 0.74 and 0.87. Also, a highly significant simple linear correlation coefficient exists between the adsorption coefficients and the OM content of the soils (r = 0.99, p < 0.001). These findings indicate the importance of the OM in linuron adsorption by the soils studied, as has also been reported by other authors (Kookana et al., 1992; Berglöf et al., 2000; Sánchez-Camazano et al., 2000a). Values for Kom (Table 2) vary within a tight range (250–335), which is in agreement with the significant correlation found between the adsorption affinity coefficients and the OM content of the soils.
Figures 2 and 3
show the desorption isotherms of atrazine and linuron in water for the five soils, together with the desorption isotherms of herbicides in the presence of the surfactant. These isotherms fit Eq. [2], with r 0.90 values for atrazine and r 0.73 for linuron. The desorption constants (Kfd and nd) calculated from Eq. [2] are shown in Table 3, together with the hysteresis coefficient (H) defined by the n to nd ratio (Gonzalez and Ukrainczyk, 1996; Morrica et al., 2000).
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The desorption isotherms of linuron (Fig. 3) also show positive hysteresis and a similar behavior to that of atrazine in all soils. Desorption coefficients and hysteresis coefficients increase with OM content (Table 3). The H coefficients range from 2.31 in Soil 1 to 41.5 in Soil 5 and are higher than those for atrazine desorption. The percentages of linuron desorbed are lower than 50%, decreasing to 2.74% for Soil 5. A significant linear correlation coefficient exists between the percentages of desorption and the soil OM contents (r = –0.98, p < 0.01).
The hysteresis phenomenon observed for herbicide desorption in water, together with the decrease in their desorption with the increase in the soil OM content, could be due to the restricted diffusion of the herbicide within soil micropores during desorption rather than to chemical transformations. Some authors (Pignatello, 1989; Scheunert, 1992) have reported that the hysteresis could be due to the type of adsorption mechanism and to phenomena such as the incorporation of the pesticide in the soil OM at sites of irreversible adsorption and/or the formation of bound residues that can only be sparingly desorbed.
Adsorption of Triton X-100 by Soils
Table 4 shows the distribution coefficients, Kd, obtained for Triton X-100 adsorption by the soils and the equilibrium concentrations, Ce, expressed in cmc. The highest Kd coefficients occur when the surfactant is initially in monomeric form (Ci = 0.75cmc, where Ci is the initial concentration). Upon increasing the concentration of the surfactant to values slightly higher than the cmc (Ci = 1.50cmc), the Kd coefficients decrease (Soils 1, 2, 3) or do not vary (Soils 4 and 5). At the high initial concentration of 50cmc, that is, when a high proportion of the surfactant must be in micellar form, a dramatic decrease in Kd is observed. The highest values of Kd always correspond to Soil 1, with the highest clay content. At the 50cmc concentration, Kd value in Soil 1 is between 7 and 40 times the values obtained in the other soils. The equilibrium concentrations after surfactant adsorption, expressed in cmc of Triton X-100 (Table 4), range between 0.07 and 0.35cmc at Ci = 0.75cmc, between 0.23 and 0.53cmc at Ci = 1.50cmc, and between 34.7 and 49.6cmc at Ci = 50cmc. These equilibrium concentration values indicate a decrease in the amount of surfactant present in solution as a result of its adsorption by the soils.
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Adsorption studies of Triton X-100 by soils have been addressed by different authors (Urano et al., 1984; Liu et al., 1991) using concentrations lower than the cmc. Adsorption has been described by nonlinear isotherms and it becomes constant for a value of the bulk solution surfactant concentration higher than the cmc (Liu et al., 1991). The interaction of Triton X-100 with soil components results by partitioning in the soil OM, owing to the hydrophobic nature of part of this compound, and by specific adsorption, through hydrogen bonds, between the polar part of the molecule and the soil minerals or OM specific groups (Edwards et al., 1992; Brownawell et al., 1997; Salloum et al., 2000). Different researchers (Sun and Jaffé, 1996; John et al., 2000) have reported that surfactant adsorption by soils initially gives rise to the formation of hemimicelles. These structures may later bind to other surfactant molecules in solution through their free hydrophobic groups, forming admicelles.
The high adsorption of Triton X-100 by Soil 1 (Table 4) can be explained by the presence of the clay mineral montmorillonite, which has high adsorption capacity (Table 1). In previous adsorption studies of Triton X-100 by montmorillonite, Salloum et al. (2000) reported intercalation of surfactant in the interlayer spacing of this mineral even at low surfactant concentrations. As a result of this adsorption process of surfactant, the organic C content and also the hydrophobic surface of the soil increase. This may favor the interaction of the pesticide with the surfactant in the soil more than with the surfactant in solution, as will be discussed below.
Competitive Herbicide Adsorption between Soil and Surfactant in the Soil–Herbicide–Water–Surfactant System
Figures 2 and 3 show the desorption isotherms of atrazine and linuron in Triton X-100 solutions at concentrations of 0.75, 1.50, 50, and 100cmc. All the isotherms obtained for atrazine (except Soil 1 at 50 and 100cmc) fit Eq. [2], with r values between 0.99 and 0.88. However, only the adsorption isotherms of linuron obtained for 0.75 and 1.50cmc solutions fit Eq. [2], with r values between 0.82 and 0.99. The desorption coefficients, Kfd and nd, calculated from this equation are shown in Tables 5 and 6, together with the H coefficients.
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Some authors (Edwards et al., 1991) have indicated increases in the solubilization of hydrophobic organic compounds in the water–surfactant system with the surfactant solution concentration. Other authors (Zheng and Obbard, 2002) have found that the solubilization of these compounds in the soil–water–surfactant system was influenced by the adsorption of surfactant by the soil. As a result, under these conditions the increase in the solubility of organic compound will be affected by its distribution between the surfactant adsorbed by the soils and the surfactant in solution. Depending on the balance between these competitive processes, the increase in the solubility of compound will occur to a greater or lesser extent.
According to the hysteresis coefficients obtained, the main effect of Triton X-100 at concentrations close to the cmc (0.75 and 1.50) is to increase the affinity of atrazine for the soil, except for soils high in OM content where the surfactant effect is to enhance the affinity of atrazine for the aqueous phase. These results indicate that at concentrations close to or slightly higher than the cmc the effect of Triton X-100 on atrazine desorption is not seen or is very small, even though at those concentrations the surfactant molecules are forming micelles and the potential solubilization of Triton X-100 micelles has been indicated for different organic compounds (Edwards et al., 1991). This is due to the decrease in the surfactant concentration that occurs in the soil–water–surfactant system as a result of surfactant adsorption by the soils studied (Table 4). The effective cmc for desorption of atrazine from the soil must be greater than that corresponding to the cmc in water and this difference varies in the different soils owing to the influence of soil characteristics on the adsorption of Triton X-100 monomers. Edwards et al. (1992) and Doong et al. (1996) have also indicated an increase in the cmc of Triton X-100 in the soil–water system due to surfactant adsorption by soil. Other authors (Sun et al., 1995; Lee et al., 2000) have also reported increases in the adsorption of hydrophobic organic compounds by soils in the presence of Triton X-100. Reports have been made (Lee et al., 2000) of increases in the adsorption coefficients regardless of the solubility in water or Kow of organic compounds at concentrations lower than the cmc, while at concentrations higher than the cmc the adsorption coefficients only increase when pollutant solubility is high.
When the Triton X-100 concentration in the soil–water–surfactant system is increased above the cmc (50 and 100cmc), the atrazine desorption constants, Kfd and nd, decrease and increase, respectively, with respect to those obtained in surfactant solution at concentrations close to cmc, except in Soil 1 (Table 5). The desorption isotherms of atrazine from Soil 1 do not fit Eq. [2] in surfactant solutions for any of the concentrations above the cmc studied. The hysteresis coefficients for the desorption isotherms obtained at 50cmc decrease for all soils, and are further reduced when the concentration is 100cmc. In Soil 5, H coefficients decrease fivefold in relation to that in water (compare Tables 3 and 5). According to these coefficients, the effect of the surfactant (except in Soil 1) is to increase the affinity of atrazine for the aqueous phase, in which Triton X-100 is in micellar form, and hence increase herbicide desorption with respect to its desorption in water. At these elevated concentrations, as was seen for 50cmc (Table 4), the effect of the very low amount of surfactant adsorbed by the soils (except by Soil 1) is almost negligible with respect to the positive effect of Triton X-100 micelles in solution.
Linuron desorption constants, Kfd and nd, in Triton X-100 solutions at concentrations of 0.75 and 1.50cmc are different for all soils when compared with those obtained in water (compare Tables 3 and 6). The desorption curves show positive hysteresis for all soils and the hysteresis coefficients decrease with respect to those corresponding to desorption in water. At the 0.75cmc concentration, the H coefficient decreases more than threefold in Soil 5. The results obtained point to a decrease in the irreversibility of adsorption under conditions in which the surfactant molecules should be present as monomers and a variation in the surfactant effect as a function of the soil OM content.
As in the case of atrazine, the surfactant effect on linuron desorption varies for the different soils. However, despite the decrease in surfactant concentration in the soil–water–surfactant system (Table 4), the hysteresis coefficients indicate that the presence of Triton X-100 increases the affinity of linuron, a more hydrophobic herbicide than atrazine, for the aqueous phase when the surfactant is present at concentrations close to the cmc. A correlation between the partition coefficient in Triton X-100 and the hydrophobicity of polycyclic aromatic hydrocarbons has been reported by Edwards et al. (1991). Doong et al. (1996) also described a greater degree of solubilization in Triton X-100 solutions of different concentrations for the most hydrophobic monocyclic aromatic compounds.
The desorption isotherms of linuron in Triton X-100 solution at concentrations of 50 and 100cmc do not fit Eq. [2]. These isotherms point to a fairly strong desorption of linuron in the first washing with the surfactant in Soils 2, 3, 4, and 5 due to the affinity of linuron for the hydrophobic core of surfactant micelles. This first desorption is followed by other smaller desorptions in successive treatments. Soil 1 is an exception to this observed desorption pattern. As was the case for atrazine desorption in Soil 1, initially an increase occurs in linuron adsorption by Soil 1 for the first washing with the surfactant, and linuron desorption begins with successive washings.
Enhanced Solubility of Herbicides in the Soil–Herbicide–Water–Surfactant System in the Presence or Absence of Micelles
Tables 5 and 6 show the enhanced solubility of atrazine and linuron in 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 or linuron desorbed in Triton X-100 solution and the percentage of these herbicides desorbed in water. For atrazine only in Soil 1 does D decrease, from 56.0 to 45.7% when the surfactant concentration increases from 0.75 to 100cmc. The greatest increase in atrazine desorption occurs when the surfactant concentration increases from 1.50 to 50cmc. At concentrations of 50 and 100cmc, the greatest desorption percentage generally occurs with the first washing with the surfactant (Fig. 2). In Soil 1, the initial effect of washing with Triton X-100 is atrazine adsorption by the soil, 37.0 and 61.3%, for surfactant concentrations 50 and 100cmc, respectively, and herbicide desorption only occurs after the second washing of soil. The trend of Soil 1 departs from that of the other soils studied owing to its high clay fraction content and the presence of montmorillonite in that fraction. Because of this, Triton X-100 is adsorbed to a greater extent at elevated concentrations by this soil than by the others (Table 4). The adsorbed surfactant causes the soil to became hydrophobic in nature, which causes the herbicide remaining in solution to be adsorbed by the soil in the first washing with the surfactant solution.
In Soil 5, with the highest OM content, an increase in desorption is observed even when the surfactant is present in monomeric form. The efficiency coefficients (E) (Table 5) in this soil vary from 1.60 in the 0.75cmc Triton X-100 solution to 3.79 in the 100cmc Triton X-100 solution. In the soils with lower OM contents than Soil 5 (Soils 2, 3, and 4), the E coefficients are negative (E < 1) at Triton X-100 concentrations of 0.75cmc and increase to 1.33 to 2.03 at Triton X-100 concentrations of 100cmc. In Soil 1, efficiency is similar to that of water at low surfactant concentrations and is negative at concentrations higher than the cmc.
The percentages of linuron desorbed in Triton X-100 solutions vary with OM content and with the surfactant concentration in solution (Table 6). An increase is observed in the percentage of desorption in all soils, even when the surfactant is present in monomeric form. The E coefficients (Table 6) range in Soil 5 from 2.33 to 18.2 when the Triton X-100 concentration increases from 0.75 to 100cmc. For Soils 2, 3, and 4, with lower OM contents than Soil 5, the E coefficients increase from 1.19 to 1.32 in the 0.75cmc Triton X-100 solution to 3.39 to 8.31 in the 100cmc Triton X-100 solution. In Soil 1, the efficiency coefficient is similar to that of water (E = 1.05) at a low surfactant concentration, and E increases to 1.34 at the 100cmc concentration although at the 50cmc concentration this coefficient is negative (E = 0.13). These results for the desorption of linuron from Soil 1 are similar to those obtained for the desorption of atrazine from the same soil and can be explained in the same terms.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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50cmc). The highest desorption occurs in the soil with an OM content of 10.3%, at a Triton X-100 concentration of 100cmc for both herbicides. However, the desorption in relation to water is much higher for linuron desorption (18.2-fold) than for atrazine desorption (3.79-fold). In Soil 1, with the highest clay content of the soils studied (29.7%) and with montmorillonite, the surfactant is adsorbed by the soil and the desorption of atrazine and linuron at high concentrations of Triton X-100 (50 and 100cmc) is lower than in water except for linuron desorption at 100cmc concentration. The results obtained provide valuable information for a better understanding of the interactions in the complex soil–herbicide–water–surfactant system and in particular between the surfactant and herbicide in solution and on the soil. |
ACKNOWLEDGMENTS |
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