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Journal of Environmental Quality 31:1226-1233 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Organic Compounds in the Environment

Anionic Polyacrylamide Effects on Soil Sorption and Desorption of Metolachlor, Atrazine, 2,4-D, and Picloram

Jianhang Lu, Laosheng Wu*, John Letey and Walter J. Farmer

Department of Environ. Sci., Univ. of California, Riverside, CA 92521

* Corresponding author (laowu{at}mail.ucr.edu)

Received for publication July 16, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Polyacrylamide (PAM) treatment of irrigation water is a growing conservation technology in irrigated agriculture in recent years. There is a concern regarding the environmental impact of PAM after its application. The effects of anionic PAM on the sorption characteristics of four widely used herbicides (metolachlor, atrazine, 2,4-D, and picloram) on two natural soils were assessed in batch equilibrium experiments. Results showed that PAM treatment kinetically reduced the sorption rate of all herbicides, possibly due to the slower diffusion of herbicide molecules into interior sorption sites of soil particles that were covered and/or cemented together by PAM. The equilibrium sorption and desorption amounts of nonionic herbicides (metolachlor and atrazine) were essentially unaffected by anionic PAM, even under a high PAM application rate, while the sorption amounts of anionic herbicides (2,4-D and picloram) were slightly decreased and their desorption amounts increased little. The impact mechanisms of PAM were related to the molecular characteristics of PAM and herbicides. The negative effects of PAM on the sorption of anionic herbicides are possibly caused by the enhancement of electrostatic repulsion by presorbed anionic PAM and competition for sorption sites. However, steric hindrance of the large PAM molecule weakens its influence on herbicide sorption on interior sorption sites of soil particles, which probably leads to the small interference on herbicide sorption, even under high application rates.

Abbreviations: HPLC, high performance liquid chromatography • PAM, polyacrylamide


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
APPLICATION of high molecular weight polyacrylamide (PAM) to soil is an effective, nonintrusive, and economical technology to enhance infiltration, stabilize soil structure, and reduce soil erosion (Barvenik, 1994; Sojka and Lentz, 1996). Polyacrylamide works by stabilizing soil surface structure and pore continuity. The increase in PAM applications in recent years drew attention to the environmental impact of PAM. Anionic PAM, used in soil systems, was reported to have low toxicity to macrofauna, edaphic microorganisms, or crop species (Barvenik, 1994; Kay-Shoemake et al., 1998). However, since addition of PAM to soil can affect soil dispersion, flocculation, aggregation (Ben-Hur et al., 1992), and chemical activity, it can potentially influence the environmental fate of pesticides or other coapplied agrochemicals.

Previous research showed that PAM treatment could reduce field losses of phosphorus and organic matter (Lentz et al., 1998), as well as transport of suspended particle-sorbed pesticides (Agassi et al., 1995) by reducing runoff volume and sediment concentration. Little information, however, is available on how PAM application will affect pesticide sorption–desorption processes. Since sorption–desorption is one of the most important factors controlling pesticide movement to ground water (Koskinen and Harper, 1990), the impact of PAM application on pesticide sorption–desorption is of great concern. Watwood and Kay-Shoemake (2000) investigated the sorption of two herbicides, atrazine and 2,4-D, on soil samples collected from the surface of PAM-treated and untreated furrow. They found that sorption of the two herbicides and desorption of atrazine were unaffected by PAM treatment while desorption of 2,4-D occurred more readily from the PAM-treated soil. Their experiment, however, investigated the effect of residual PAM on herbicide sorption–desorption and degradation. Other important data, such as how PAM impacts herbicide sorption kinetics and by which mechanisms PAM affects herbicide sorption, are still unknown.

Two nonionic herbicides, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] and atrazine (2-chloro-4-ethylamino-6-isopropylamine-s-triazine), and two anionic herbicides, 2, 4-D (2,4-dichlorophenoxyacetic acid) and picloram (4-amino-3,5,6-trichloropicolinic acid), were selected to investigate the impact of PAM on sorption behavior. All of them are used commonly (George, 1994), and the first three are among the most widely used pesticides (USEPA, 2000). Their sorption–desorption and leaching behaviors on soils have been extensively studied, for example, metolachlor by Obrigawitch et al. (1981) and Peter and Weber (1985), atrazine by Clay and Koskinen (1990) and Seybold and Mersie (1996), 2,4-D by Grover (1973) and Wilson and Cheng (1978), and picloram by Farmer and Aochi (1974) and Watson et al. (1989). Sorption–desorption characteristics of these herbicides related to soil organic matter, pH, clay mineralogy, salinity, and other soil properties can be found in these documents. Briefly, for nonionic metolachlor and atrazine, organic matter content is the primary factor affecting the magnitude of sorption (Peter and Weber, 1985; Laird et al., 1992), while for anionic 2,4-D and picloram, besides soil organic matter, soil pH also has significant influence on their sorption (Barriuso et al., 1992; Farmer and Aochi, 1974). When soil pH is lower than the herbicide pKa, apparent enhancement of sorption occurs.

The objective of this study was to further investigate the sorption–desorption behavior of these four herbicides as affected by PAM treatment. The results could be used to appropriately evaluate PAM environmental impacts and help to improve PAM application methodology.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Herbicides and Soils
Metolachlor (purity 96.1%), atrazine (purity 98%), and 2, 4-D (purity 98%) were purchased from Chem Service (West Chester, PA). Picloram (purity 99.4%) was purchased from Sigma Chemicals (St. Louis, MO). All herbicides were used as received. Granular anionic polyacrylamide, with an average molecular weight of 10 to 15 million g mol-1, 21% NH2 group substituted by OH group, was provided by Celanese Corporation (Louisville, KY). All other chemicals used were analytical grade or high performance liquid chromatography (HPLC) grade.

Two soils, a Linne loam (fine-loamy, mixed, thermic Calcic Pachic Haploxeroll; Chula Vista, CA) and an Arlington sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralf; Riverside, CA), were used in this study. Soil samples were collected from the surface (0 to 15 cm). They were air-dried and ground to pass through a 1-mm sieve. Their textural and chemical properties are described in Table 1 . Particle-size analysis was determined by hydrometer method (Gee and Bauder, 1986) and organic matter content by the 450°C combustion method (Davies, 1974).


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  		Table 1. Textural and chemical properties of the two soils used in this study.

 
Sorption–Desorption
Sorption experiments were conducted using the batch equilibration technique. Both herbicides and PAM were prepared in a 0.01 mol L-1 CaCl2 matrix. For the sorption of herbicide in the absence of PAM, soil was added to herbicide solutions with various concentrations (10 to 50 µmol L-1, approximating relevant field application rates) in a series of 20-mL glass centrifuge bottles. The soil to solution ratio (g g-1) was 1:4 for metolachlor and atrazine, and 1:2 for 2,4-D and picloram. The bottles were capped and reciprocally shaken for 24 h at 25 ± 2°C. Preliminary experiments showed that sorption equilibrium could be achieved in this time period for all the herbicides. After shaking, the bottles were centrifuged for 20 min at 3000 x g, and 2 mL supernatant was withdrawn for HPLC analysis. The amount of herbicide adsorbed was calculated from the difference between the initial and final solution concentration.

Desorption and kinetics experiments were performed for the samples having an initial herbicide concentration of 40 µmol L-1. For desorption studies, the remaining slurry in the centrifuge bottles was refilled to the original volume by adding 2 mL 0.01 mol L-1 CaCl2 solution and reequilibrating for 24 h. The procedure (withdrawing the supernatant, replacing it with 0.01 mol L-1 CaCl2 solution, and reequilibration) was then repeated five times. For sorption kinetic studies, the bottles were shaken for a time period ranging from 0.25 to 24 h (different solid–solution contact time) before centrifuging and withdrawing supernatant for HPLC analysis. All the experiments were performed in triplicate.

For the sorption experiments in the presence of PAM, the initial procedure was modified to allow a preequilibration period for PAM sorption on soil. Soil samples were first added to PAM solution, then dispersed using a vortex mixer, and shaken for 36 h for PAM to reach sorption equilibrium (Lu et al., 2002) before addition of herbicide solution. The herbicide solution was twice concentrated to get the same initial concentration as in the absence of PAM. For the comparative experiments using oxamic acid (with a molecular structure similar to PAM), the same procedure was used. Oxamic acid solution was also prepared in a 0.01 mol L-1 CaCl2 matrix, and the pH was adjusted to 7.0 with 0.001 mol L-1 NaOH solution. The sorption amounts of oxamic acid on soil were estimated from total organic carbon analysis of the supernatants (Dohrmann [Santa Clara, CA] DC-80 carbon analyzer).

The range of PAM application rates used in the experiments to investigate the effects of PAM on herbicide sorption was from 20 to 500 mg kg-1 (mass of PAM to mass of soil). The highest rate was about half of the saturation sorption amount of PAM on the Arlington soil and one-third that of the Linne soil. Under the batch experimental conditions described above, almost all the PAM in aqueous solution was adsorbed onto the soil since no observable PAM in supernatants was detected by the N-bromination method (Lu and Wu, 2001) at the end of 36 h of equilibration. Therefore, the application rate of PAM is equal to the amount of PAM added to the soil.

Sorption and desorption isotherm parameters were estimated using the linear form of the Freundlich equation:

where x/m is the amount of herbicide sorbed to soil (µmol kg-1), Ce is the equilibrium concentration in solution (µmol L-1), and Kf and n are empirical constants (Kfs and ns will hereafter refer to sorption, and Kfdes and ndes to desorption). The term Kf is an index of sorption capacity, and n is an index of sorption intensity (Weber and Miller, 1989).

Since the n value varies with individual isotherm, comparison of Kf values will be meaningless. Statistical evaluation of the differences between the sorption isotherms with and without PAM treatment was done by the paired Student's t test on the means of sorption amounts.

Dialysis
Spectra/Pro CE disposal dialyzers (Spectrum Laboratories, Rancho Dominguez, CA) with a high molecular weight cutoff of 100000 Da were used in this study. Preliminary studies showed that PAM molecule could not pass through this dialysis membrane (detected by a modified N-bromination method with a detection limit of 0.2 mg L-1; Lu and Wu, 2001), while herbicides could readily diffuse through the membrane and reach equilibrium in 24 h. The dialyzers were thoroughly washed with deionized water before use. Aliquots of 5 mL of herbicide and PAM (50 µmol L-1 herbicide and 20 mg L-1 PAM in a 0.01 mol L-1 CaCl2 matrix, aged four days before dialysis) mixture were pipetted into dialysis tubes. The dialyzer was floated in a narrow 40-mL-capacity glass bottle containing 25 mL 0.01 mol L-1 CaCl2 solution. A magnet stirring was performed and the bottles were capped and wrapped with aluminum foil to minimize possible photolysis. After 24 h, the concentration of herbicide inside and outside of the dialysis tubing was measured by HPLC and the difference between them was assumed to be PAM-bound herbicide concentration.

Herbicide Analysis
Determination of herbicide concentration in supernatant was performed with a Hewlett–Packard (Palo Alto, CA) 1090 HPLC equipped with an auto-injection system and a diode-array detector. The column was a reverse phase Adsorbosphere HS C18 column (4.6 by 250 mm i.d., particle size 5 µm; Alltech Associates, Deerfield, IL), injection volume was 20 µL, and the wavelength of the detector was set at 220 nm. The mobile phase was a mixture of acetonitrile and water that was acidified with 1 g L-1 phosphoric acid. The ratio of acetonitrile and water (v/v) was 80:20 for metolachlor, 60:40 for atrazine and 2,4-D, and 40:60 for picloram. The flow rate was maintained at 1.0 mL min-1 for all herbicides. Under these conditions, the retention time of metolachlor, atrazine, 2,4-D, and picloram was 5.37, 5.10, 4.69, and 4.86 min, respectively.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Effects of Polyacrylamide on Sorption Kinetics of Herbicides
The adsorbed amounts of herbicide on a control and a PAM-treated Linne soil (PAM application rate was 80 mg kg-1) at different solid–solution contact times are shown in Fig. 1 . For the sake of clarity, the high adsorptive herbicides (metolachlor and atrazine, on Fig. 1a) are plotted separately from the low adsorptive herbicides (2,4-D and picloram, on Fig. 1b) and only data from the first 8-h period are presented. These results indicate that PAM treatment slightly retarded the sorption rate of herbicides, especially in the first 4 h. Herbicides can reach sorption equilibrium (i.e., the difference of sorption amounts is less than 5% between two consecutive measurements) in 4 to 8 h on the natural soil, but it takes 8 to 12 h to reach this equilibrium on the PAM-treated soil.



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  		Fig. 1. Sorption kinetics of herbicides ([a] metolachlor and atrazine, [b] 2,4-D and picloram) on the natural and PAM-treated Linne clay loam. The PAM application rate was 80 mg kg-1, and initial herbicide concentration was 40 µmol L-1. Each data point is the mean of three replicates and the error bar is its standard deviation.

 
In general, the sorption rate of an herbicide is governed by two processes: transport of adsorbate from the exterior bulk solution to accessible sorption site in the interior soil matrix and formation of sorptive bonding. The former is a diffusion process, and the latter is related to sorption mechanisms, such as the activation energy of sorptive bonds. Either of these processes, however, can be the rate-limiting step (Pignatello and Xing, 1996). Since no bonding interaction between herbicides and PAM in aqueous solution occurred, and only slight or no change of herbicide sorption capacity in the presence of PAM was observed (see results below), PAM did not appear to measurably alter herbicide sorptive mechanisms on soil. Therefore, it is unlikely that PAM will interfere with the transport process of herbicides. The retardation of herbicide sorption is more likely due to the slower transport of herbicides from exterior bulk solution to accessible sorption sites in PAM-treated soils. It was observed that addition of PAM led to visible aggregation of soil particles. Polyacrylamide can be strongly adsorbed onto the surface of soil particles and can keep them from dispersion. Due to its strong sorptive affinity and low diffusion ability, the adsorbed PAM molecules may form a film on the surface of soil particles, which retards the penetration of herbicide molecule into interior pores, especially to sorption sites inside some of the inkbottle-shaped pores.

Effects of Polyacrylamide on Equilibrium Sorption Amounts of Herbicides
Sorption–desorption isotherms for the four herbicides on natural and PAM-treated soils (PAM application rate was 80 mg kg-1) are shown in Fig. 2 . The fitted Freundlich parameters, Kf and n, are presented in Table 2 . All the correlation coefficients were greater than 0.99 (n = 5) for sorption isotherms, and greater than 0.97 (n = 6) for desorption isotherms. The Kf and n values for all the herbicides on the two soils fell within the range of those previously reported values (Seybold and Mersie, 1996; Grover, 1973; Farmer and Aochi, 1974).



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  		Fig. 2. Sorption–desorption isotherms of herbicides ([a] metolachlor, [b] atrazine, [c] 2,4-D, [d] picloram) on natural and PAM-treated soils. Each data point is the mean of three replicates and the error bar is its standard deviation. The solid line is the sorption isotherm and the dashed line is the desorption isotherm. The PAM application rate was 80 mg kg-1.

 

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  		Table 2. Freundlich sorption–desorption isotherm parameters for metolachlor, atrazine, 2,4-D, and picloram in the Linne and Arlington soils. The polyacrylamide (PAM) application rate was 80 mg kg-1.

 
The data in Fig. 2 and Table 2 indicate that the impact of PAM on herbicide sorption behavior depends on their chemical forms in aqueous solution. The sorption and desorption of nonionic herbicides, metolachlor and atrazine, exhibited no significant difference between natural and PAM-treated soils (paired Student's t test, P < 0.05; also see data in Fig. 2 and Table 2), indicating that the sorption–desorption capacity of nonionic herbicides was almost unaffected by anionic PAM. However, sorption of anionic herbicides (2,4-D and picloram) was slightly decreased by PAM treatment, while their desorption was slightly increased (Fig. 2). These results correspond to those from an earlier report (Watwood and Kay-Shoemake, 2000), except that desorption of anionic herbicides was only slightly increased by PAM treatment. In Watwood and Kay-Shoemake's experiments, significantly higher desorption of 2,4-D on PAM-treated soil compared with PAM-untreated soil was observed. This difference might be attributed to the difference in desorption experiment protocols. In their experiments, desorption isotherms were determined by single desorption of several samples with varying initial concentrations, while our desorption isotherms were obtained by consecutive desorption of one sample. Moreover, our soil samples were subjected to different PAM-treatment regimes and application rates.

The variable effects of PAM on sorption of different herbicides can be explained by their different sorption mechanisms. The sorption of anionic herbicides on soils must overcome the electrostatic repulsion from the negative charge of the soil surface. The presorbed anionic PAM molecules cause an increase of negative charge in the soil surface, which enhances the repulsive electrostatic forces and decreases sorption of anionic herbicide. Moreover, possible competition for sorption sites between similar carboxyl groups in 2,4-D, picloram, and PAM molecule might reduce the sorption of these anionic herbicides. However, for nonionic herbicides, none of these possibilities exists, which explains the lesser impacts observed here.

Effects of Polyacrylamide Application Rate on Herbicide Sorption
The typical application rate of PAM in furrow irrigation practices is 0.9 kg ha-1 (Sojka and Lentz, 1996). Usually it was applied in the advance stream in furrow irrigation water at a low concentration of 10 mg L-1. Assuming that PAM only penetrates 1 to 2 mm into soil and the water in furrows wets about 25% of the field surface area (Sojka and Lentz, 1996), the concentration of PAM in the surface soil exposed to flowing water could be as high as 30 to 60 mg kg-1 soil. Since PAM degrades very slowly in soil with a rate of about 10% per year (Azzam et al., 1983), consecutive irrigation may lead to an even higher concentration. In this study, we used a PAM application rate of 80 mg kg-1 soil to evaluate its impact on herbicide sorption behavior.

Compared with small molecular agrochemicals such as pesticides, PAM has a very high sorption affinity on the soil matrix (Lu et al., 2002), and once adsorbed to soil, very little desorption occurs (Nadler and Letey, 1989). These two aspects contribute to its low mobility in soils. As a result, PAM may accumulate on the surface soil exposed to irrigation water containing PAM, especially at the furrow head. Exact PAM concentration on field soils is difficult to estimate due to nonuniform distribution of PAM in the soil, and no method is available to extract PAM from soils. Thus, varying PAM application rate over a wide range is useful for thoroughly assessing PAM impacts on herbicide sorption behavior in the field.

The sorption amounts of herbicides on two test soils with an initial concentration of 40 µmol L-1 were plotted against PAM application rate in Fig. 3 . As shown in Fig. 3, addition of PAM to soil had almost no effects on equilibrium sorption amount of nonionic herbicides (metolachlor and atrazine) over the investigated application rate range. In contrast, the equilibrium sorption amounts of anionic herbicides (2,4-D and picloram) decreased slightly as PAM application rate increased, which was attributed to the increasing electrostatic repulsion between anionic herbicide and negative charge of soil surface and the competition for sorption sites as PAM application rate goes up. However, the impacts of PAM were not significant. Sorption amounts were reduced by less than 20% even under the extremely high PAM application rate, which was hardly used in the field.



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  		Fig. 3. Effects of PAM application rate on herbicide ([a] metolachlor and atrazine, [b] 2,4-D and picloram) sorption. The initial herbicide concentration was 40 µmol L-1. Each data point is the mean of three replicates and the error bar is its standard deviation.

 
Mechanisms of Polyacrylamide Impacts
There are many possible ways in which PAM may influence herbicide sorption behavior. Physically, PAM application results in an agglomeration of soil particles and reduces dispersion, thus decreasing the external soil surface area and the amount of available sorption sites for herbicides. It has been observed that the sorption capacity of a soil depends upon the degree of dispersion (Huggenberger et al., 1973; Abu-Zreig et al., 1999). Also, PAM can enhance water infiltration (Lentz and Sojka, 1994), which might allow adsorbed herbicide to be more easily flushed off the soil matrix. Chemically, the sorption of PAM on the soil matrix may (i) increase herbicide sorption by enhancing hydrophobic partition, bonding interactions (such as Van Der Waal's or dipole–dipole interaction), and additional sorption sites for some herbicides, as presorbed PAM may be viewed as a means for increasing soil organic matter content; (ii) decrease herbicide sorption by occupying some sorption sites and making them inaccessible to herbicide; and (iii) decrease the sorption of anionic herbicide by increasing the negative charge of soil particles. The net effect of PAM treatment on sorption will depend on the soil and herbicide properties.

To explain the mechanisms of PAM impacts, two aspects related to the molecular characteristics of PAM must be considered. First, as both the PAM and herbicide molecules contain –CO–, –NH2, and –COOH groups, they can possibly form stable complexes through hydrogen bonding or other mechanisms that will lead to synergism during sorption. Second, the large molecular size of PAM may hinder its effects on the sorption of small herbicide molecules, as some of their sorption occurs inside the micropore of the soil matrix that is not accessible to PAM.

In this research, an equilibrium dialysis procedure was used to investigate the possible interaction between PAM and the four test herbicides in aqueous solution. Moreover, a small molecular chemical, oxamic acid, which has the same –CONH2 and –COOH groups as PAM, was selected to compare their impact on herbicide sorption. The dialysis technique has been shown to be an effective tool for verifying the association of herbicide and high molecular dissolved organic matter (Carter and Suffet, 1982; Lee and Farmer, 1989). Complexes between herbicide and high molecular substance (e.g., humic acid and polymer) should result in a larger concentration of herbicide inside the dialysis tube than outside. The dialysis results of the mixtures of 40 mg L-1 PAM and 50 µmol L-1 herbicide (in a 0.01 mol L-1 CaCl2 matrix) were presented in Table 3 . For the four herbicides, no measurable differences between inside and outside concentrations were observed, suggesting that no stable bonding happened between these herbicides and anionic PAM in aqueous solution. This result helps to exclude the possibility that PAM could enhance sorption of the four test herbicides, which agrees with our above-mentioned experimental results (Fig. 2 and Table 2).


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  		Table 3. Equilibrium concentration of herbicides inside and outside dialysis tube. Numbers in parentheses are standard deviations.

 
The sorption isotherms of four herbicides on the Linne soil preequilibrated with oxamic acid solution were plotted in Fig. 4 . The sorption amount of oxamic acid on the soil was approximate 42 mg kg-1. The data in Fig. 4 showed that oxamic acid slightly decreased the sorption of nonionic herbicides (metolachlor and atrazine) and considerably decreased the sorption of anionic herbicides (2,4-D and picloram), especially in the high concentration zone. Considering that the adsorbed amounts of oxamic acid on the soil were only about half of the application rate of PAM (80 mg kg-1), one can conclude that oxamic acid has a much greater influence on herbicide sorption. The molecular structure of oxamic acid and the repeating unit of PAM molecule are similar. Both consist of amide groups and carbonyl groups. Their chemical interference on herbicide sorption, such as competition for accessible sites, should be similar. Hence, the difference of their effects on herbicide sorption can be attributed to the large molecular size of PAM.



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  		Fig. 4. Sorption isotherms of herbicides ([a] metolachlor, [b] atrazine, [c] 2,4-D, [d] picloram) on the Linne soil in the presence of oxamic acid. The sorption amount of oxamic acid on the soil was 42 mg kg-1. Each data point is the mean of three replicates and the error bar is its standard deviation.

 
Earlier studies suggested that high molecule PAM had limited ability to diffuse into soil aggregates (El-hardy and Abd El-hardy, 1989) and its sorption was largely restricted to the external surface (Malik and Letey, 1991). However, the structure of the sorption matrix in soil, such as humic acid, has been postulated to be loose and open and contain voids or holes of different molecular dimensions (Schnitzer and Khan, 1972). Sorption of small herbicide molecules can occur both on the external surface and interior surface of soil particles (Pignatello and Xing, 1996). Therefore, it appears that the steric hindrance of the large PAM molecule nullifies its influence on herbicide sorption on interior sites, which possibly leads to the limited effect on herbicide sorption compared with oxamic acid.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The change of physical and chemical characteristics of surface soil due to application of anionic PAM could potentially impact sorption behavior of herbicides. Polyacrylamide treatment slowed down the sorption process of herbicide on soils. A longer time was required for herbicides to achieve sorption equilibrium on PAM-treated soils than on natural soils. The impact of PAM treatment on the amounts of equilibrium sorption depends on the molecular characteristics of herbicides. For nonionic herbicides, PAM treatment has no measurable effects on sorption and desorption amounts, even under high PAM application rates; while for anionic herbicides, PAM treatment slightly decreased sorption amounts and increased desorption amounts. The slower sorption kinetics and reduced sorption capacity of herbicides due to PAM treatment could potentially facilitate their movement through soils, but only with a small magnitude.

The similarity of functional groups in PAM molecule with anionic herbicides possibly causes the decrease in herbicide sorption amounts on PAM-treated soils. However, steric hindrance of large PAM molecules to interior sorption sites of the soil matrix weakens its influence and leads to only slight impacts on herbicide sorption even under a high PAM application rate.

It is important to point out that the results from batch experiments may overestimate the effects of PAM treatment in the field, as only a small portion of soil will be affected by PAM during irrigation. For example, in furrow irrigation, PAM only affects the surface soils that are exposed to irrigation water. Besides, PAM only penetrates a few millimeters under the soil surface, but herbicides may go much deeper. The majority of soil volume involved in herbicide movement in the entire field is unaffected by PAM treatment.

Herbicides and/or pesticides could be carried off the field by getting adsorbed to the sediment and/or dissolved in the runoff water. The principal purpose of PAM treatment was to reduce the fine-particle erosion and cut down runoff-water volume (Lentz et al., 1998). These advantages could negate the slightly increasing mobility of herbicides and/or pesticides. Additional studies should be conducted at the field scale to appropriately evaluate the PAM impact on herbicide runoff.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome