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Published in J. Environ. Qual. 32:2200-2206 (2003).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Organic Compounds in the Environment

Size Distribution of Organic Matter and Associated Propiconazole in Agricultural Runoff Material

Qinglan Wu*,a, Gunnhild Riisea and Ruben Kretzschmarb

a Dep. of Soil and Water Sciences, Agricultural Univ. of Norway, P.O. Box 5028, 1432-Ås, Norway
b Inst. of Terrestrial Ecology, ETH Zurich, Grabenstrasse 3, CH-8952 Schlieren, Switzerland

* Corresponding author (wu_qinglan{at}yahoo.com).

Received for publication October 11, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SAMPLES AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sorption and desorption characteristics of propiconazole (1-[[2-(2,4-dichlorophenyl)-4-propyl-1, 3-dioxolan-2-yl] methyl]-1H-1,2,4-triazole) to different particle/aggregate-size fractions of agricultural runoff material were investigated. Emphasis was put on clay and colloidal size fractions to evaluate their role as potential sorbents and carriers for this pesticide. The runoff material was separated into size fractions ranging from 2 mm to ca. 15 nm by wet sieving, sedimentation, centrifugation, and membrane ultrafiltration. Each fraction was characterized by its organic C content and C/N ratio. Distinctive sorption properties of clay-sized particles and colloids were investigated. The obtained size fractions differed significantly in their organic C concentration, C/N ratio, and sorption properties to propiconazole. Organic matter was mainly associated in aggregates >2 µm. Binding of propiconazole to this coarse fraction made up 80% of the sorbed propiconazole. The distribution coefficient between solid and aqueous phases increased with decreasing particle size. The colloidal fraction (<0.16 µm) exhibited the highest sorbtivity, with a distribution coefficient of 113 L kg-1, which was more than four times higher than that in the bulk sample (27 L kg-1). The fraction <2 µm represented 8% of the total sample weight, but contributed to 20% of the sorbed propiconazole. Strong hysteresis was observed for the sorption–desorption of propiconazole on the runoff material. Under dilution very little sorbed propiconazole will be released into the water phase. Due to its high sorbtivity and mobility and the strong sorption–desorption hysteresis, particles in the fraction <2 µm can be important carriers of propiconazole in runoff suspensions with high sediment load.

Abbreviations: Kd, distribution coefficient of a compound between aqueous and soil solid phases • Koc, Kd–value normalized to the weight fraction of soil organic carbon • NOM, natural organic matter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SAMPLES AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
POLLUTION OF AQUATIC systems through runoff from agricultural fields is a major environmental problem worldwide. Pesticides both in suspension and incorporated into depositional sediments are considered potential pollutant sources (Rostad, 1997; Gilliom et al., 1999; Schulz and Liess, 2001). Propiconazole (Fig. 1) is a widely used fungicide, which is frequently detected in aquatic systems (Levine et al., 1999; Castillo et al. 2000; Ludvigsen and Lode, 2001). Growing environmental concern over this pesticide is due to its long persistence (Bromilow et al., 1999) and its ecotoxicological effects on fish, invertebrates, and algae (Aanes and Bækken, 1994; Kaellqvist and Romstad, 1994). In a previous work by Riise et al. (2001), sorption of propiconazole to agricultural surface soil was investigated. Due to the high affinity of propiconazole to particulate materials, an important role of particles in the fate and behavior of propiconazole was predicted.



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  		Fig. 1. Chemical structure of propiconazole and its water solubility (S), acidity (pKa), 1-octanol/water partition coefficient (Koc), and volatility.

 
Most studies concerning the fate of pesticide runoff have focused on sorption of pesticide to whole soil material. Recently, binding of pesticides to different size fractions has received increasing attention, in particular for pesticides with high hydrophobicity (Gao et al., 1998; de Jonge et al., 2000; Leonard et al., 2001; Riise et al., 2001). The distribution of pesticides among different size fractions in runoff material is important, because particles of different size exhibit different sorbtivities to pesticides. Moreover, particles of different sizes differ in their settling velocity and therefore their transport distances and deposition patterns. In addition, organic matter attached to particles of different sizes was found to have different rates of decomposition (Hassink, 1995), which may influence the release of propiconazole associated with organic matter, and therefore its bioavailability and toxicity. The finer particles in the clay-sized and colloidal fractions are of particular interest, because of their large specific surface area and high stability in suspension, and their potential role in facilitating transport of contaminants (e.g., McCarthy and Zachara, 1989; de Jonge et al., 1998; Kretzschmar et al., 1999).

Natural organic matter (NOM) is regarded as the main sorbent for hydrophobic pesticides in soils. The close relationship between sorption of pesticides and content of organic C in soil has led to the development of the Koc coefficient (Koc = Kd/foc), where the distribution coefficient (Kd) is normalized to the weight fraction of organic C, foc (e.g., Chiou et al., 1979; Kile et al., 1999). In soils, organic matter in different particle-size fractions differs in concentration and chemical composition (Monrozier et al., 1991; Schulten and Leinweber, 2000) and also in its Koc values for pesticide sorption (de Jonge et al., 2000; Riise et al., 2001). Agricultural runoff material is enriched in NOM compared with the original surface soil due to selective erosion of the smaller particle-size fractions (Ghadiri and Rose, 1993; Wan and El-Swaify, 1997). The sorption properties of the eroded organic matter, especially that associated with finer fractions, might be of fundamental importance for the mobility of pesticides.

For studies concerning NOM size fractions, several size fractionation methods have been used. Some authors use chemical dispersion agents (e.g., sodium hexametaphosphates) and/or high energy ultrasonication to achieve a complete disruption of aggregates (Wu et al., 1993; Schmidt et al., 1999; de Jonge et al., 2000), while other authors make great efforts to keep natural aggregates as undisturbed as possible (Stemmer et al., 1998; Riise et al., 2001). For the separation between the dissolved and particulate matter, methods such as adding CaCl2 as coagulation agent, centrifugation at high speed, and membrane filtration have been used.

Our objective was to investigate the size distribution of NOM in runoff material and the ability of each size fraction to bind propiconazole. We intended to develop a size fractionation method that could cover a wide size range of runoff materials from 2 mm down to ca. 15 nm in diameter, and under conditions as natural as possible, without disrupting the microaggregates. The separation method described here is a combination and a modification of the methods developed by Borkovec et al. (1993), Stemmer et al. (1998), and Riise et al. (2001). The size fractions obtained were characterized by their content of organic C and C/N ratio. Sorption–desorption properties of size fractions and the bulk sample were determined by batch experiments. Binding of propiconazole to different size fractions and their role in pesticide transport and deposition in aquatic systems were discussed.


   SAMPLES AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SAMPLES AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Runoff Material
Runoff material was collected from the inlet of an artificial pond, where eroded material detached from an agricultural field in Ørje, Norway, was trapped. The sample was air dried, gently crushed by a roller, sieved to <2 mm, and stored at room temperature. Material >2 mm was discharged. The runoff material had a pH value of 6 (measured in 0.01 M CaCl2), a clay content of 0.25 kg kg-1, and an organic C content of 15.5 g kg-1.

Isolation of Particle-Size Fractions
The runoff material was fractionated into eight size fractions by wet sieving, sedimentation, centrifugation, and membrane filtration techniques (Fig. 2) . For fractions <2 µm, the size was the equivalent spherical diameter calculated using Stoke's law. The average hydrodynamic diameters of these fractions were additionally determined in triplicate by dynamic light scattering at 90° on a goniometer instrument (ALV/SP-125S/N30, ALV-Laser, Germany) equipped with a 1-W Krypton laser (Innova 300, Coherent, Germany) operating at a wavelength of 647 nm.



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  		Fig. 2. Flow-chart for the size fractionation method.

 
To 1000 mL pure water (Millipore water) 50 g of runoff material was added and shaken for 48 h at a rate of 125 rounds per minute (Edmund Eühler SM25 Tübingen, Germany). For the size fractionation without shaking, the same amount of runoff material was added. The cylinder was closed and turned gently end-over-end by hand for 10 times. The suspension was left to stand for 24 h and then wet sieved to separate the three largest size fractions: 250 to 2000 µm (I), 63 to 250 µm (II), and 20 to 63 µm (III). The suspension <20 µm was collected in a cylinder (1 L) and used for sedimentation analysis under gravitational force to get the fraction 2 to 20 µm (IV). The supernatant in the cylinder (<2 µm) was decanted to a glass bottle, transferred into four 250-mL centrifuge bottles (Nalgene, USA), and centrifuged at 3000 x g for 15 min (Beckman Centrifuge Type J2-21M, Rotor JS-7.5) to get the fraction 0.16 to 2 µm (V). The supernatant (<0.16 µm) was decanted, transferred into eight 35-mL centrifuge tubes, and centrifuged at 20000 x g for 30 min (Rotor JA 20.1) to get the fraction 0.03 to 0.16 µm (VI). The supernatant <0.03 µm was filtrated through a membrane with a cut off at 10 kDa (ca. 15 nm). The concentrated sediment fractions V and VI were stored at 4°C until further use. The entire fractionation scheme is summarized in Fig. 2.

An aliquot of each size fraction was dried at 60°C for gravimetrical and chemical analysis. The contents of C and N were determined with a CHN-analyzer (CHN-1000, LECO). In addition, the concentrations of total organic C were measured in the solutions using a TOC-analyzer (TOC-5000, Shimadzu).

We used one sedimentation step (under gravitation or centrifugation force) to separate the fractions 0.16 to 2 µm and 0.03 to 0.16 µm. The obtained size distribution within each size fraction may not have a clear cut-off at both ends of the size range. However, for the size interval we chose, the distributions in each size fraction are sharp enough to be distinguished from each other. The average hydrodynamic diameter measured by dynamic light scattering technique lies at 0.65 µm for fraction 0.16 to 2 µm and 0.25 µm for fraction 0.03 to 0.16 µm. The specific surface area of different fractions, which is an important parameter for sorption, differs also significantly among size fractions as shown by our earlier measurement on some soil samples using the same size fractionation procedure (Borkovec et al. 1993).

Since the dry weights and organic C contents of the two smallest fractions (<10 kDa and <0.03 µm) were very low and not significantly different from one another (<10 kDa: 60 ± 10 mg/L dry wt., 3.6 ± 0.5 mg/L organic C; and <0.03 µm: 52 ± 10 mg/L dry wt., 4.2 ± 0.3 mg/L organic C), we neglected the particulate matter in the fraction 10 kDa to 0.03 µm and regard the fraction <0.03 µm, the supernatant after centrifugation at 20000 x g for 30 min, as the aqueous phase. Centrifugation at 20000 x g for 30 min was also conducted to separate the solid and aqueous phases for the sorption–desorption experiment.

Size Distribution of the Hydrogen Peroxide–Treated Sample
An aliquot of the runoff material (20 g) was treated with 30% H2O2 to remove organic matter. When the color of the sediment sample turned lighter, the suspension was heated to 90°C to evaporate the rest of H2O2 and then cooled down to room temperature. The suspension was transferred to a 1-L glass cylinder and filled with 0.1% sodium polyphosphate (Merck) to 1 L. The particle-size distribution was determined according to Gee and Bauder (1986).

Determination of Sorption and Desorption Equilibrium
The sorption and desorption isotherms of propiconazole for the bulk sample were determined by batch experiment according to the OECD Guideline 106 (OECD, 1997).

Propiconazole labeled with 14C in the triazole ring (purity >95%, specific activity of 88.73 Ci kg-1, Inst. of Isotopes Co., Ltd, Budapest) was used for sorption experiments. In the higher concentration range (initial conc.: >0.27 mg L-1), 14C-labeled propiconazole was diluted with nonlabeled propiconazole (purity >97%).

A fixed amount of air-dried soil sample (2.0 g, size fraction <2 mm) was weighed into a Teflon centrifuge tube (Nalgene, USA) and pre-equilibrated with 5 mL of 0.01 M CaCl2. After 16 h end-over-end shaking, the suspensions were equilibrated with 5 mL of 0.01 M CaCl2 spiked with 14C-labeled propiconazole in a concentration range between 0.027 and 0.54 mg L-1. After a further shaking for 24 h, the suspensions were centrifuged at 20000 x g for 30 min (Beckman Centrifuge, Model J2-21M). Analyses of 14C were run on 1-mL samples taken from the upper portion of the centrifuge tube (Packard Tri-Carb 4530 liquid scintillation counter).

After the sorption experiment, additional 4 mL of supernatant were taken from each centrifuge tube and replaced by 5 mL of 0.01 M CaCl2 to continue with desorption experiments. After 48 h end-over-end shaking, separation of solid and aqueous phases and analyses of 14C were conducted as described above.

For fractions V (0.16–2.0 µm) and VI (0.03–0.16 µm), 5 mL of suspensions of known sediment concentration were transferred to 10-mL Teflon centrifuge tubes (Nalgene, USA) and mixed with the same volume of 0.01 M CaCl2 spiked with 14C-labeled propiconazole. Sorption and desorption experiments were performed under the same conditions as for the bulk sample.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SAMPLES AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Size Distribution of Aggregates and Particles in Runoff Material
Both shaking and H2O2 treatment of the runoff material had a major influence on the measured particle–aggregate-size distribution (Fig. 3) . In samples without shaking, 46.5% of the sediment mass were located in the coarse fraction 250 to 2000 µm and only 3.8% in the fraction <2 µm. The fraction 0.03 to 0.16 µm made up <0.2% and the dissolved matter <0.1% of the total sample weight. After shaking for 48 h, the percentage of the coarse fraction 250 to 2000 µm decreased from 46.5 to 27.4%, whereas the fraction <2 µm only increased from 3.8 to 8.9%. Microaggregates between 2 and 250 µm made up the major part in the suspension. The removal of natural organic matter by H2O2 treatment disrupted the aggregate structure and caused a further shift in the size distribution toward smaller clay particles. The coarse mineral fraction (250–2000 µm) contributed only to 2.2% of the total mass, while the clay fraction <2 µm accounted for 27% in the H2O2 treated sample.



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  		Fig. 3. Particle–aggregate-size distribution of agricultural runoff sediment in a sediment–water suspension with a concentration of 50 g L-1. Samples with H2O2 treatment were size fractionated down to 2 µm.

 
The result in Fig. 3 indicates that runoff material in the suspension is composed predominantly of aggregates of mineral particles stabilized by organic matter. Macroaggregates are mainly located in the size fraction 250 to 2000 µm, which are distinctly less stable against mechanical disruption than the fractions <250 µm. Through shaking, macroaggregates break into the more stable microaggregates mainly between 2 and 250 µm in diameter, which again consist of organo–clay complexes of even smaller size.

The size distribution of runoff sediment in water suspensions is quite sensitive to mechanical disruption. To get reproducible results, the samples were shaken for a relatively long time period (48 h), followed by 24 h standing for conditioning the suspension. Procedures such as ultrasonication, adding of chemical dispersion agents, and coagulation agents were avoided to keep the size distribution as intact as possible. Indeed, the measured aggregate-size distribution is only valid for runoff materials that have been dried and resuspended during the separation procedure. On the other hand, mechanical disruption of macroaggregates, drying, wetting, and so forth will also occur in the field. Even though the size fractionation procedure disturbs the aggregate structure to some extent, the size distribution of the breakdown products can, however, still be used to characterize the runoff sediment. The mass percentage of the size fraction <2 µm can be used as a measure for the susceptibility of sediment to release clay-sized particles, which are very important for pesticide binding and transport.

For the nonshaken size fractionation, it is difficult to get reproducible results, because uncontrolled mechanical disruption may occur during wet sieving or gentle homogenization of the suspension. In the following experiments presented, only size fractions obtained after 48 h shaking were used. The recovery rate for the sediment mass was 97.6 ± 1.5%. The standard deviation of replicates for each size fraction was smaller than 1.6% for the fractions <2 µm and up to 7.2% for the coarser fractions.

Table 1 shows that the average hydrodynamic diameters of the finer size fractions <2 µm, <0.16 µm or <0.03 µm only changed slightly by removal of organic matter and additional dispersion with ultrasonication. This indicates that particles in these size fractions consist rather of primary particles stabilized by organic matter than aggregates.


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  		Table 1. Average hydrodynamic diameter of particle-size fractions measured by dynamic light scattering in water suspension before and after treatment with H2O2 and additional ultrasonication.

 
The average hydrodynamic diameters of the size fractions <0.16 µm and <0.03 µm measured by dynamic light scattering were larger than the upper cutoff limits calculated using Stoke's law. This may be attributed to the difference between the effective density of organo–clay complexes and the assumed specific density of 2.65 g cm-3 for mineral particles. In addition, the average hydrodynamic diameter of polydisperse samples measured by dynamic light scattering is always biased toward the larger particles, which strongly contribute to the total scattering intensity. Finally, the nonspherical shape of sediment particles can be another reason for this discrepancy. For the fraction <10 kDa the light scattering intensity was too low to be detected, which is consistent with the fact that the particle concentration in this fraction is very low.

Distribution of Organic Matter among Size Fractions
The distribution of organic C among aggregate-size fractions of the runoff material is presented in Table 2. The C concentration was higher both in the coarser and the finest fraction, and exhibited a minimum in the silt-size fraction 2 to 20 µm. This trend is consistent with previous observations made for some Alfisols by Monrozier et al. (1991) and for sediments by Gao et al. (1998). Other authors (Schulten et al., 1993; Stemmer et al., 1998) found that the distribution of organic C in different size fractions depends on soil type. The coarse-size fraction may contain larger amounts of less decomposed plant debris contributing to a higher concentration of organic C. On the other hand, the higher C concentration of the finer-size fraction may be due to its large specific surface area and the high sorption to natural organic matter. Therefore, it is possible that soil characteristics such as humification degree and content of clay minerals influence the distribution of organic C among different particle-size fractions.


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  		Table 2. Concentrations of organic C, total N, and C/N ratios in a size-fractionated runoff material not subject to H2O2–treatment. The deviation from the mean of two replicates is given in bracket.

 
The C/N ratio was highest in the coarse fraction 250 to 2000 µm with a mean value of 9.5 and decreased with decreasing size down to 4.9 for the fraction 0.03 to 0.16 µm. The variation in C/N ratio may again reflect the different degree of chemical decomposition of organic matter associated with different size fractions. Schulten et al. (1993) explained the lowered C/N ratio of smaller fraction size by increased content of N-containing compounds in organic matter and decreased content of lipids and lignin. An extremely low C/N ratio of about 5 for colloidal fractions (<0.1 µm) was also observed by Stemmer et al. (1998) for several surface soils. Strong sorption of organic N and inorganic N to clay particles was suggested to be the reason for the low C/N ratio.

The distribution of organic C among size fractions in percentage of total organic C of the runoff material is shown in Fig. 4 . More than 90% of the total organic C was associated with size fractions between 2 and 2000 µm. The fraction 0.16 to 2.0 µm contained only 6% of the total organic C and <1% was bound to colloidal particles (0.03–0.16 µm). The amount of organic C in the fraction 10 kDa to 0.03 µm is under detection limit of our analytic method (<1 mg C L-1) and is therefore neglected. The dissolved organic C in fraction <0.03 µm made up 0.3% of the total organic C.



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  		Fig. 4. Distribution of organic C in size fractions of the runoff sediment, expressed as percentage of total organic C.

 
Binding Capacity of Different Size Fractions
The sorption and desorption isotherms of propiconazole to the total runoff material are depicted in Fig. 5 . Both isotherms exhibited a linear shape within the tested concentration range (0.001–0.04 mg L-1). Comparison of sorption and desorption isotherms indicates a strong sorption–desorption hysteresis. The distribution coefficient for desorption is far higher than that for the sorption, and are 50.3 and 26.7 L kg-1, respectively. Only 3% of the sorbed pesticide is desorbed from the solid phase during 48 h desorption period in a single desorption step. Sorption–desorption hysteresis has been observed in many cases and is usually explained by slow sorption–desorption kinetics (nonequilibrium) and restriction because of diffusion processes (Kan et al., 1994; Altfelder et al., 2000). The strong hysteresis during desorption could result in overestimation of dissolved propiconazole if only sorption coefficient was used. The particle-bound propiconazole will largely remain in particulate form, when the runoff suspension is diluted by rainwater or when it enters rivers or lakes.



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  		Fig. 5. Sorption and desorption isotherms of propiconazole for the bulk sediment sample; Cs: sorbate concentration in solid phase and Ceq: sorbate concentration in aqueous phase; sediment/water ratio 1:5, temperature 20°C.

 
Similar experiments were conducted with different particle-size fractions, and the resulting Kd values for sorption and desorption are reported in Table 3. The results show that the sorption capacity for propiconazole increases with decreasing particle size. The sorption coefficients Kadsd, as well as desorption coefficients Kdesd for the fraction 0.03 to 0.16 µm, were four times larger than that for the bulk sample. Also, the Koc values for the finer-size fractions were much higher than for the bulk sample, which is consistent with the results obtained by Riise et al. (2001) for a silty clay soil from southeastern Norway. This may arise from the different chemical composition of the organic matter as indicated by the changing C/N ratio of the different size fractions. It is also possible that the larger specific surface area of mineral particles in submicrometer range additionally contributes to the higher sorption capacity (Riise et al., 2001). Gao et al. (1998) found that, among different size fractions of a sediment sample, the clay-sized fraction had the highest affinity to hydrophobic pesticides, but the Koc value of each size fraction was not significantly different from one another.


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  		Table 3. Distribution coefficient for the sorption (Kadsd) and desorption (Kdesd) of propiconazole in bulk runoff sediment and its size fractions. The deviation from the mean of two replicates is given in parentheses; Koc is calculated using the equation: Koc = Kadsd/foc (foc = weight fraction of organic C).

 
According to calculations based on the particle-size distribution (Fig. 3) and sorption characteristics (Table 3), >80% of the sorbed propiconazole were associated with the fractions >2 µm, which usually settle rapidly from the runoff stream and can be incorporated into depositional sediments. In contrast, pesticides bound to particles <2 µm remain suspended in water and can travel over large distances, even under relatively low water velocities. The fraction 0.16 to 2.0 µm contributes only 8% to the sediment mass, but accounts for >20% of the sorption of propiconazole to the runoff sediment. The finest fraction (0.03–0.16 µm), although it has the highest sorbtivity (Table 3), accounted only for 0.4% of the sorbed amount of propiconazole, because this fraction made up only 0.1% of the total sediment mass (Fig. 3). However, processes breaking the aggregate structure may release more colloidal or clay-sized particles, and thereby increase the importance of these fractions in binding and transporting pesticides. The concentrations of colloidal particles and macromolecules in the fraction <0.03 µm were below the detection limits of our analytical methods (10 mg L-1 for particles; 1 mg C L-1 for organic C). Even though the sorption coefficient might be high, the colloids in fraction <0.03 µm, accounting for <0.02% of the total sediment mass, will contribute little to the total sorbed propiconazole. The dissolved part of propiconazole obviously is very mobile and can be transported as surface runoff or drainage water. The importance of the dissolved form in pesticide transport depends largely on the distribution of pesticides between the dissolved and particle-bound forms.

Pesticide Distribution between Dissolved and Particle-Bound Form
The percentage of pesticide associated with particles in suspension is not only dependent on the Kd–value but also on the concentration of sediment. By using the linear sorption equilibrium model, the relationship between the percentage of particle-bound pesticide (C0 - Ceq)/C0 and the mass concentration of sediment Cm (kg L-1) can be established in Eq. [1]:

[1]
where C0 (mg L-1) is the initial concentration and Ceq (mg L-1) is the equilibrium concentration of the pesticide in water phase. This relationship is illustrated in Fig. 6 , which shows that the percentage of particle-bound pesticide increases with increasing sediment concentration (or sediment/water ratio).



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  		Fig. 6. Amount of pesticides sorbed to sediment in relation to distribution coefficient (Kd) and sediment concentration in suspension; curves are calculated based on linear sorption equilibrium; the figure on the right side is in an enlarged scale range.

 
In heavy sediment sludge or in depositional sediment where the sediment/water ratio is high, pesticides are mainly bound to particles. In agricultural runoff water, however, the sediment concentration usually is <10 g L-1, which means that pesticides with a Kd value <10 are mainly in dissolved form, while for chemicals with much higher Kd values (>10000), up to 90% of pesticides are bound to particles.

According to calculations using Eq. [1], a Kd value of 27 L kg-1 (Tab. 3) and a sediment/water ratio of 1:1 (1000 g L-1), 96% of propiconazole will be bound to the investigated runoff material. In runoff water with a sediment concentration of 10 g L-1, 21% of propiconazole will be associated with the sediment particles, which can significantly affect the fate and behavior of the pesticide. When sediment concentration in runoff water is as low as 200 mg L-1, propiconazole will mainly be in a dissolved form, and the influence of particles on the transport is small. The transport is then controlled by the concentration of dissolved propiconazole.

For processes where desorption of pesticide predominates, sorption–desorption hysteresis should be considered. Desorption may occur when pesticides sorbed to surface soil is carried into runoff water during heavy rainfall events, when sediment suspension containing pesticide is diluted along the water course, or when bottom sediment is resuspended and diluted by bulk water. One should keep in mind that the Kddes value for desorption process can be much higher than that for the sorption (Kdads). Desorption coefficient should be used with care for estimating the percentage of particle-bound pesticide, especially for pesticide that shows strong sorption–desorption hysteresis. The Kddes value is valid only for the conditions under which it was determined, because the determined Kd value can vary in dependence of experimental protocol, volume of replaced solution, initial sorbate loading, and equilibration time (Altfelder et al., 2000). For propiconazole the desorption kinetics is very slow, and we expect that a large part of propiconazole will remain on the particle surface.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 SAMPLES AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The developed fractionation method provides size fractions with significant differences, both in their concentrations and compositions of organic matter, and also in the ability to bind propiconazole.

The agricultural runoff material investigated consists predominantly of macro and micro aggregates rather than primary soil particles. The macroaggregates can easily be disrupted by shaking into microaggregates in the range between 2 and 250 µm, which are very stable in water. Up to 80% of the sorbed propiconazole are bound to aggregates between 2 and 250 µm, which will probably slow down the environmental spreading of the pesticide due to the rapid settling of coarse material. The clay-sized and colloidal fractions are very likely primary organo-mineral particles with relatively high stability in water suspension. They exhibit a much higher sorbtivity than the bulk sample. Because of their high sorbtivity, mobility, and the strong sorption–desorption hysteresis, these fractions can be important carriers of organic micropollutants. However, its importance in binding and transporting propiconazole is very much dependent on the concentration of these small particles and processes that contribute to an enrichment of smaller size fractions.


   ACKNOWLEDGMENTS
 
We thank The Research Council of Norway for funding the projects: "The role of natural organic matter (NOM) for the transport of pesticides, no. 141207/720" and "Ecotoxicological effects of particle-bound pesticides, no. 132121/110." We appreciate the technical assistance of I. E. Dahl, I. Digernes, and M. Pettersen at Dep. of Soil and Water Sciences, NLH, Norway and K. Barmettler at Institute for Terrestrial Ecology, ETH Zurich, Switzerland.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SAMPLES AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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JEQ 2003 32: 1931-1938. [Full Text]  




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