Colloid-facilitated transport of pesticide in undisturbed soil columns
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Pergamon Phys. Chem. Earrh, Vol. 23, No. 2, pp. 187-191, 1998 0 1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain 0079-1946/98 $19.00 + 0.00
Colloid-Facilitated Transport of Pesticide in Undisturbed Soil Columns
H. de Jonge, 0. H. Jacobsen, L. W. de Jonge and P. Moldrup2
Department of Soil Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark *Environmental Engineering Laboratory, Department of Civil Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Alborg, Denmark
Received 25 April 1997; accepted 15 December 1997
Abstract. The purpose of this work was to verify whether facilitated transport enhances the vertical movement of a relatively strongly adsorbing pesticide, and to study whether ionic strength and pH affects the pesticide and particle transport. Experiments were carried out with 20*20 cm undisturbed soil columns taken from the topsoil (sandy loam, typic Hapludalf) from a field under normal cultivation near Rogen, Denmark. The selected pesticide, prochloraz, was applied to the surface as a pulse in solution. Facilitated transport was significant, but was not dominating the transport of the pesticide: about 10% of the pesticide was bound to particles with diameter d >0.24 urn. Preferential flow and particle transport were the two most important factors determining the amount of pesticide leached. Decreasing ionic strength and increasing pH promoted leaching of particles and pesticide.
counter-ions around charged surfaces, and the surface charge of organic and inorganic particles. Generally, a decreasing ionic strength and increasing pH is reported to lead to increased colloid transport, which is in qualitative accordance with theory (McDowell-Boyer et al. 1986; Liu et al. 1995). Thus, the pH and ionic strength were expected to influence the colloid mobilisation in this soil, and the objectives of the present work were: i) to verify whether facilitated transport by in-situ mobilised colloids affects the vertical movement of the pesticide prochloraz in undisturbed topsoils; ii) to study the influence of ionic strength and pH, the latter as affected by ammonia application, on the particle mobilisation and the pesticide transport.
0 1998 Elsevier Science Ltd. 2 Materials and Methods
The risk of leaching of strongly adsorbing compounds has been assumed to be low, but such compounds have been reported to move large distances in soils and sediments. This has motivated research towards facilitated transport and preferential flow, two mechanisms that could enhance transport rates. Facilitated transport and preferential flow are interrelated because colloid transport velocities are higher in macropores, where preferential flow occurs (Ryan and Elimelech 1996). Facilitated transport is a complex process dependent on the sorption properties of the solute, colloidal surface properties, solute chemistry, pore structure, and hydraulic conditions. Ionic strength and pH are key factors for colloidal transport as they affect, respectively, the extent of the diffuse double layer of
Correspondence ro : H. de Jonge, Danish Institute of Agricultural Sciences, Dept. of Soil Science, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele. Denmark.
The undisturbed soil samples were collected in 20*20 cm stainless steel rings from a depth of 2-22 cm from the Regen field station near Aarhus, Denmark. The rings were pushed into the ground, excavated, and the ends of the cylinders were trimmed. The columns were stored at 2C until further use. The soil was classified as a Typic Hapludalf, and the textural classification was a sandy loam with a clay, silt and sand content of 15.7, 27.8 and 54.1%, respectively. The organic carbon content was 1.5% and the CEC of the soil was 0.157 Mol, kg-.
Before the start of each experiment, the samples were saturated with a 0.01 M CaCls solution and subsequently drained and equilibrated to a pressure of -30 cm (relative to the top of the column) for 3 days. The solute input system consisted of a solution reservoir, a peristaltic pump with variable speed control (Masterflex, Cole Parker Instr., Chicago, Ill), a sample injector (Rheodyne 3725-038, Cotati, Ca) equipped with a 50 ml stainless steel sample loop, a 100 ml injection syringe (SGE, Ringwood, Australia), stainless steel tubing (1.02 mm ID, Waters,
188 H. de Jonge et al. ,, la, lb, Ic. 1d:lowscale
0 3 6 24 27 TIME [hour]
I- O 3 6 24 27
TIME [hour] 0 3 6 24 27
Figure 1. The turbidity of the effluent for the three different treatments. The axis break denotes a flow interruption of 17 hours. the arrow indicates the change of the applied solute to the ammonia solution. Note the different scales on the turbidity axis.
Milford, Ma), and a stainless steel irrigation head equipped with 29 05*16mm needles (Becton Dickinson Microlance3, Dublin, Ireland) with 30 mm spacing. The column was automatically slowly rotated throughout the experiments at 3 revolutions per hour to obtain a homogeneous irrigation pattern at the surface. The soil column was placed on a steel screen so that an atmospheric pressure lower boundary condition was obtained. The effluent was collected through a glass funnel in a beaker glass, before subsequently measurements on the effluent were performed. The columns were irrigated at an intensity of 10 mm/hour.
The effluent was sampled throughout the experiment and analysed for turbidity (Hach 2100 AN, Loveland, Co) and pesticide concentration. Pesticide concentrations were quantified on a liquid scintillation analyser (Packard 2250 CA, Downers Grove, Ill). Both a total pesticide and a particle-bound pesticide concentration were measured, being subsamples from the same effluent sample. The operationally defined solution phase was taken from the supernatant after centrifuging a 10 ml subsample for 10 min at 54208, resulting in a lower cut-off particle diameter of 0.24 pm. The particle-bound pesticide concentration was then calculated as the difference of the total amount and the solution phase.
Prochloraz (IUPAC name: iV-propyl-N-[2-(2,4,6- trichlorophenoxy)ethyl] imidazole-1-carboxamide) has a log &,=4.4, and a maximum aqueous solubility of 34 mg/l. The linear sorption distribution coefficient for the silty loam used in this study was KJ = 53, and log K,, = 3.5. Ring labelled 14C prochloraz obtained from International Isotopes (Munich, Germany). Input solutions containing 5 mg/l pesticide were prepared by mixtures of a C stock solution and a non-labelled pesticide stock solution.
The pH and ionic strength were varied in the experiments, giving 3 treatments (l-3) with 4 replicates (a- d) as shown in Table 1. In the first three hours the irrigation solution was either 0.01 M CaC12 (treatment 1 and 2, pH=6.0, EC=0.199 S/m) or a low ionic conductivity
solution (treatment 3, pH=7.82, EC=2.24 mS/m) mimicking the ionic strength of rainwater and containing 0.012 mM CaC12, 0.015 mM MgC&, and 0.121 mM NaCl. At time t=60 minutes, a 50 ml solute pulse was added containing 5 mg!l of pesticide. This pulse also contained a background electrolyte: 0.01 M CaClz (treatment 1 and 2), or 0.012 mh4 CaCl*, 0.015 mM MgC12, and 0.121 n&l NaCl (treatment 3). After 180 minutes, the irrigation solution was either kept at 0.01 M CaClz (treatment l), or switched to a 0.11 M N&OH (treatment 2 and 3, pH=l 1 .O, EC= 0.03 S/m, referred to as ammonia solution). The irrigation was continued until t=7 hours, and started again from t=24 hours until t=27 hours. The columns were finally irrigated with a dye solution (Brilliant Blue R250, 1 g/l) for 1 hour, and dissected in 4 layers of 5 cm. The tracer coloured areas were drawn on a sheet.
replicates: treatment 1 treatment 2 treatment 3 4 (la-Id) 4 (2a-2d) 4(3a-3d)
inigation solution t=o-3hr: pulse input t=l hr:
pulse input t=3hr:
irrigation solution t=3-7hr: irrigation solution t=2%27hr:
0.01 M CaCIz 0.01 M CaClz rainwater
50 ml, 5 mg/l SO ml, 5 mg/l 50 ml, 5 mg/l prochloraz, prochloraz, prochloraz 0.01 M CaBrs 0.01 M CaBr2
50 ml, 0. I1 M NH40H, 0.01 M NH4Br
0.01 M CaC12 0.11 MNH40H 0.11 M NH4OH
0.01 M CaC12 0.11 MNH40H 0.11 M NH.+OH
Table 1. The experimental design of the column experiments
3 Results and Discussion
The dye experiments showed that the water transport occurred in macropores, earthworm burrows, root channels,
Colloid Facilitated Transport of a Pesticide in Undisturbed Soil Columns 189
r 1 a: %w scale lb, lc. Id: high scale
0 3 6 24 27
28. 2~. 2d: low scale 2b: hiph scale
3a. 3b. X. 3d: high scale
0 3 6 24 27 0 3 6 24 27
TIME [hour] TIME [hour]
Figure 2. The pesticide concentrations of the effluent for the three different treatments. The axis break denotes a flow interruption of 17 hours, the arrow indicates the change of the applied solute to the ammonia solution. Note the different scales on the pesticide concentration axis.
and other preferential flow paths. The area fraction dyed by the Brilliant Blue colour tracer was smaller than 0.1, as shown in Table 2. Some transport along the column wall occurred in 5 of the 12 columns (columns la, Id, 2a, 2d, 3~). This is believed to be due to random variation of the spatial distribution of the macropores in the column and not to be related to disturbances due to sampling. This is supported by the fact that these columns did not systematically leach higher amounts of pesticide. Table 2 also presents the static soil physical data that were collected. The bulk density was rather constant with a standard deviation of 3.2% around the mean of 1.57 Mg/m3.
The nephelometric turbidity (NTU) is proportional to particle mass, as was shown by other experiments with this soil (Jacobsen et al. 1997). For treatment 1, large particle concentrations were initially measured after which concentrations fell to a low baseline value. The turbidity rise after the flow interruption is for this soil probably related to detachment and/or diffusion processes (Jacobsen et al. 1997). The change from O.OlM CaC12 to the ammonia solution in treatment 2 caused a smooth and sigmoidal increase of the particle concentration in the effluent. Obviously, the detachment rate of particles increased due to the change in the pH. For treatment 2, the turbidity remained approximately constant after the flow interruption. Hence, detachment and diffusion are no longer the rate- limiting factors for particle mobilisation under the high pH conditions.
column pb+ ES 8, 5 een dyed area no.
[Mg/m] [m/m] [m3/ms [m/m] f-1 1
la n.d. + + n.d. n.d. n.d. ,025 lb 1.65 ,378 ,326 ,323 ,009 lc 1.59 ,400 ,336 ,340 .04 I Id I .63 ,386 .339 ,345 .070 2a 1.46 ,450 n.d. ,309 .087 2b 1.51 .429 n.d. ,320 .059 2c 1.58 ,402 .325 ,331 .I01 2d 1.55 .415 ,353 ,361 ,052 3a I .5.5 ,416 .356 ,362 .086 3b 1.58 .405 ,355 ,360 ,059 3c 1.58 .403 .342 ,348 ,090 3d 1.60 ,395 ,330 ,343 .05 I
Average: 1.51 .407 .338 ,341 ,061 std. dev.: 0.051 .019 .05 1 ,017 ,026
dry bulk density; total porosity, assuming a solid phase density of 2.65e3 kg/m; B initial water content; water content after the experiment; average dyed area from depth planes of 5, 10 and 15 cm; + + not determined.
Table 2. Soil physical parameters and the area fractions wetted by the Brilliant Blue colour dye.
Treatment 3 mobilised more particles in the first three hours than treatments 1 and 2 (Figure 1). The initial concentrations are the same as treatments 1 and 2, but the baseline level is higher (note the different scale for column 3~). This can be interpreted as follows: the amount of
190 H. de Jonge et al.
particles that initially can be transported is the same, as the initial conditions are equal for all columns. After some time, the lower EC induces expansion of the double layer around clay particles, causing higher detachment rates. The turbidity rise after the pH change is more abrupt and the patterns are erratic in contrast to the relatively smooth curves of treatment 2.
The variation in the pesticide leaching behaviour among replicates was large. Another paper discusses more in detail this dataset, including factors like pH, and bromide transport (de Jonge et al., 1997). The two factors, other than treatments effect, that were found to be most strongly related to the total amount of pesticide leached were the bromide peak arrival volume (R= -0.715), and the average turbidity (R=0.663). The total pesticide breakthrough curves showed early breakthrough and extensive tailing, confirming the heterogeneity of the water transport in macropores and other preferential flow domains (Figure 2). In treatments 2 and 3, the pH change caused secondary peaks or more gradual increases of the total and particle- bound pesticide concentrations. This is in contrast to treatment 1 where steadily decreasing concentrations were recorded during the first seven hours of the experiment. For columns 2a and 2c, the pesticide concentrations after the solution switch even exceeded the initial peak concentrations. A very high secondary peak is observed for column 3a corresponding to the particle-bound pesticide. The arrival of this surge of particle-bound pesticide coincided with the steep turbidity rise.
The fraction of pesticide bound to particles larger than 0.24 urn was typically less than 10% of the total amount. For some columns negative concentrations were obtained when the concentrations were close to the detection limit. The breakthrough of the particle-bound pesticide has an erratic pattern with multiple breakthrough peaks. In treatment 3, the pH change caused secondary peaks of the particle-bound pesticide, but this was not the case for treatment 2. This could be explained by the more pronounced and efficient particle transport in treatment 3 (see also Figure 1).
1 e-4 lo-3 le-2 lo-l
SOLUTION CONCENTRATION C, [mg/l]
Figure 3. Pesticide isotherm in effluent together with the independently measured isotherms of the clay and silt fraction of this soil (solid line).
The particle-bound pesticide concentrations were higher than would be predicted by the adsorption isotherms. In Figure 3, the pesticide concentrations are presented as adsorbed versus solution phase together with independently measured isotherms for clay and silt size separates of this particular soil (Petersen et al, 1996). Only columns from treatment 1 are shown in Figure 3 because these can be directly compared to the isotherms that were also measured in a 0.01 M CaCIZ solution. The solid phase concentrations can only be smaller or at maximum equal to the value predicted by the & when considering classical breakthrough of a sorbing compound. Given the high soibsolid ratios in the column and the short residence time of the pesticide in the column, from this classical viewpoint it would be anticipated that the particle adsorbed concentrations would lie below the isotherm. However, the particle-a...