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Journal of Environmental Quality 31:1234-1239 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Organic Compounds in the Environment

Picloram and Napropamide Sorption as Affected by Polymer Addition and Salt Concentration

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

Department of Environmental Sciences, Univ. of California, Riverside, CA 92521

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

Received for publication October 4, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Polymer application to soil is a growing practice to improve soil physical properties and reduce soil erosion. Polymer addition can potentially influence herbicide and pesticide sorption in soil. The one-point distribution coefficient Kd values of two herbicides in the absence and presence of each of 10 polymers (7 polyacrylamides and 3 polysaccharides) were determined by the batch equilibrium method. The results showed that nonionic napropamide [2-(alpha-naphthoxy)-N,N-diethyl propionamide] sorption was essentially unaffected by the presence of any of the polymers. The influence of polymers on anionic picloram (4-amino-3,5,6-trichloropicolinic acid) sorption depends on the charge characteristics of polymers and salt concentrations in the solution. Electrostatic interaction and competition for sorption sites are two primary underlying mechanisms for the polymer influence. At low salt concentration, the increased picloram sorption in the presence of both cationic and anionic polymers was attributed to different electrostatic interactions and polymer partitioning between soil and solution phases. At high salt levels, the presence of polymers had either no influence or a slightly negative influence on the picloram sorption, which was attributed to competition for sorption sites. In field conditions, it is more likely that polymers have no or a slightly negative influence on herbicide sorption due to the presence of salts.

Abbreviations: DI, deionized • PAM, polyacrylamide


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
SINCE THEIR INTRODUCTION for agricultural use in the early 1950s (Weeks and Colter, 1952), synthetic polymers have often been used as soil conditioners to improve soil physical properties. Many polymers, such as polyacrylamide (PAM) and polysaccharide, are used to enhance infiltration, stabilize soil structure, and reduce soil erosion (Barvenik, 1994; Lentz et al., 1992). Polyacrylamide treatment of irrigation water has recently become a fast-growing conservation technology (Sojka and Lentz, 1996). By the year 1999, about 400000 ha irrigated lands were PAM-treated in the United States (Sojka and Surapaneni, 2001). In addition, polymers have also been introduced to soil in pesticide formulation to reduce pesticide leaching (Wietersen et al., 1993; Hall et al., 1998). As the amount of PAM application increases, concerns for its influence on the environment are rising.

Naturally occurring polymers, such as dissolved organic matter mainly derived from humic-like materials, have been shown to influence the pesticide sorption–desorption on soil and sediments (Madhun et al., 1986; Lee et al., 1990). Synthetic polymers, because they can significantly affect soil dispersion, flocculation, aggregation (Ben-Hur et al., 1992), and chemical activity, potentially influence the environmental fate of pesticides and other coapplied agrochemicals. Earlier research has shown that PAM treatment can 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. Soil treatment with anionic PAM could kinetically reduce the sorption rate of herbicides and slightly decrease the sorption amounts of anionic herbicides (Lu et al., 2002b). Desorption of some herbicides, such as 2,4-D (2,4-dichlorophenoxyacetic acid), occurs more readily in soils containing PAM residues (Watwood and Kay-Shoemake, 2000). However, previous research focused only on the most frequently used PAM polymer, which has a molecular weight of 10 to 15 million g mol-1 and moderate negative charge (15 to 25% NH2 group substituted by OH group). Research on other PAMs and commercially available polymers has not been reported. Since these PAMs have different charge characteristics and molecular structures, knowledge of the influence of PAM properties on pesticide environmental behavior is necessary to appropriately evaluate their environmental impact and help to minimize their negative influence on the pesticide transport potential to surface and ground water.

Two herbicides, a nonionic napropamide and an anionic picloram, were selected to investigate the influence of polymers on their sorption behavior. Picloram has a pKa of 3.4 and predominately exists in the anionic form at neutral pH. Both of the herbicides are commonly used (George, 1994). Their sorption–desorption and leaching behaviors in soils have been extensively studied. Sorption–desorption characteristics of these two herbicides are related to soil organic matter, pH, clay mineralogy, salinity, and other soil properties. Briefly, for nonionic napropamide, organic matter content is the primary factor affecting the magnitude of its sorption (Wu et al., 1975; Gerstl and Yaron, 1983), while for anionic picloram, in addition to soil organic matter, soil pH also has a significant influence on its sorption (Farmer and Aochi, 1974; Nearpass, 1976). When soil pH is close to or lower than its pKa, picloram sorption increases noticeably.

Anionic PAM sorption was greatly enhanced by the dissolved salts in soil solution (Lu et al., 2002a). Sorption of herbicides, such as anionic picloram, nonionic atrazine (2-chloro-4-ethylamino-6-isopropylamine-s-triazine), and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide], was also found to increase at higher salt levels (Farmer and Aochi, 1974; Spongberg and Lou, 2000). To investigate the influence of polymers on herbicide sorption, the role of salts should be properly considered.

The objective of this study was to investigate the influence of different polymers on the sorption of two herbicides on soil materials at different salt levels and to explain their impact mechanisms based on the soil properties and the charge characteristics of polymers and herbicides.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Soil, Herbicides, and Polymers
Picloram (purity 99.4%) was purchased from Sigma Chemicals (St. Louis, MO). Napropamide (purity 99.0%) was purchased from Chem Service (West Chester, PA). Both were used as received. The soil used in this study was an Arlington sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralf) from Riverside, CA. Soil samples were collected from the surface (0 to 15 cm), air-dried, and ground to pass through a 1-mm sieve. The soil had the following characteristics: a particle-size distribution of 75.8% sand, 12.7% silt, and 11.5% clay (the hydrometer method; Gee and Bauder, 1986); organic matter content of 13.6 g kg-1 (450°C combustion method; Davies, 1974); soil pH in 0.005 M CaCl2 of 7.6 (1:1 soil to solution ratio); and cation exchange capacity of 13.9 cmol kg-1. The 1:2 soil to deionized (DI) water ratio equilibrium supernatant had an electrical conductivity (EC) of 0.32 dS m-1, and a sodium adsorption ratio of 0.68.

Ten polymers with different molecular structures and charge characteristics were used in this study. All of them have been tested or used for agricultural purposes (Ben-Hur et al., 1990). Seven of them are PAMs and the remaining three are derivatized natural polysaccharide polymers (guar). The seven synthetic PAMs included three anionic (PAM 2J, 21J, and 40J), one nonionic (Percol 333), and three cationic (Percol 747, 763, and 757) polymers. The anionic PAMs have very high molecular weight (10 to 15 x 106 g mol-1) and their NH2 groups are substituted by OH groups at 2 (2J), 21 (21J), and 40% (40J) to create various magnitudes of negative charge. The very low, negatively charged PAM (2J) is usually considered a nonionic polymer. The cationic PAMs are copolymers of quanternary acrylate salt and acrylamide at various ratios used to create low cationic charge density (Percol 747), medium cationic charge density (Percol 763), and high cationic charge density (Percol 757). The three guar polymers are medium-charge anionic (T-4246), low-charge cationic (CP-14), and medium-charge cationic (T-4141). They have lower molecular weights (0.2 to 2 x 106 g mol-1) than the PAMs. The schematic structure of the guar polymers can be found in earlier literature (Ben-Hur and Letey, 1989). The nonionic and cationic PAMs were provided by Allied Colloids (Suffolk, VA), and the anionic PAMs and guar polymers were provided by Rhone-Poulenc (Louisville, KY).

Preparation of Low-Salt Soil Sample
In order to evaluate the role of salt concentration in herbicide sorption, batch experiments were carried out on the same soils after a portion of their salt was removed. Low-salt soil samples were obtained by consecutive DI water washing of the natural soil. Four hundred grams of the Arlington soil were added to 800 mL DI water in a polyethylene plastic bottle and stirred for 12 h with a mechanic stirrer. Then, the turbid soil suspension was transferred to centrifuge tubes and centrifuged for 20 min at 1000 x g. The clear supernatant was discarded and the remaining slurry was transferred back to the plastic bottle by DI water washing. About 500 mL DI water was supplemented, and the above procedure was repeated three more times. After that, the soil sample was air-dried and ground to pass through a 1-mm sieve. With these treatments, the electrical conductivity of the 1:2 soil to DI water ratio equilibrium supernatant was lowered to 0.056 dS m-1 from the initial 0.32 dS m-1.

Herbicide and Polymer Sorption
The one-point sorption distribution coefficient (Kd) of herbicides was measured by the batch equilibration technique. Picloram or napropamide solutions were prepared in DI water or 0.005 M CaCl2 mixed with 200 mg L-1 of each of the 10 polymers. Initial concentration of the herbicides was 20 mg L-1. Six grams of soil were added to 12-mL herbicide solutions in a series of 25-mL glass bottles. The bottles were capped, dispersed with a vortex mixer, and reciprocally shaken for 24 h at 25 ± 2°C. Preliminary experiments showed that sorption equilibrium could be achieved in this time period for both herbicides. Then, the bottles were centrifuged for 10 min at 3000 x g, and 2 mL of supernatant was withdrawn for HPLC analysis. The amount of herbicide adsorbed was calculated from the difference between the initial and final solution concentrations. The sorption experiment for each treatment was repeated four times.

The same procedure with no herbicides in the polymer solutions was also conducted to investigate the partition of polymers between the solution and soil matrix. The concentrations of polymers were estimated from total organic carbon (TOC) analysis of the supernatants (Dohrmann [Santa Clara, CA] DC-80 carbon analyzer). Soil TOC background was subtracted by blank treatment. Preliminary experiments showed that the presence of herbicides (with the same initial concentration as in the experiments with herbicide sorption) had no measurable influence on polymer sorption.

The low salt level in the equilibrium sorption supernatants was achieved by adding polymer and herbicide solutions prepared in DI water to the leached Arlington soil. Medium and high salt levels were achieved by adding polymer and herbicide solutions prepared in DI water and in 0.005 M CaCl2 solutions to the natural Arlington soil.

The Kd values were calculated by dividing the sorbed concentration by the solution concentration: Kd = Cs/Ce, where Cs is the herbicide sorbed (mg kg-1) and Ce is the equilibrium concentration in solution (mg L-1).

Statistical evaluation of the differences between herbicide sorption in the presence and absence of polymers was done by the Student's t test on the Kd values.

Dialysis
Spectra/Pro CE disposal dialyzers (Spectrum Laboratories, Rancho Dominguez, CA) with a molecular weight cutoff of 25000 Da were used in this study. Preliminary studies showed that all the PAMs and >95% of guar polymers could not pass through the dialysis membrane (detected by total organic C analysis), while both herbicides readily diffused through the membrane and reached equilibrium in 48 h. The dialyzers were thoroughly washed with DI water before use. Five-milliliter aliquots of a mixture of each herbicide and each of 10 polymers (100 mg L-1 picloram or 40 mg L-1 napropamide) and 200 mg L-1 polymer at different salt levels (aged for four days before dialysis) were pipetted into dialysis tubes. The dialyzer was floated in a tube-shaped 70-mL-capacity glass bottle containing 55 mL herbicide- and polymer-free solution. The outside solution had the same salt level as the solution inside the dialysis tube. A magnetic stirrer was applied and the bottle was capped and wrapped with aluminum foil to minimize possible photolysis. After 48 h, the concentrations of herbicide inside and outside the dialysis tube were measured by HPLC. For picloram, dialyses were conducted in DI water (low salt level), in 0.001 M CaCl2 + 0.0005 M NaCl (medium salt level, with approximately the same electrical conductivity and sodium adsorption ratio [SAR] for the 2:1 water to soil ratio equilibrium supernatant of the Arlington soil), and in a 0.005 M CaCl2 (high salt level) matrix. For napropamide, no dialysis in salt solutions was performed since no measurable concentration difference was observed in DI water.

The difference in herbicide concentration inside and outside the dialysis tube was described by the interaction ratio (Ri), which is defined in the same way as the complexation ratio by Clapp et al. (1997):

where Cin is the concentration of herbicide inside the dialysis tube, and Cout is the concentration of herbicide outside the dialysis tube. Associative interaction between herbicide and polymer leads to a positive Ri, while repulsive interaction between herbicide and polymer results in a negative Ri.

Herbicide Analysis
Herbicide concentration analysis in supernatant was conducted on a Hewlett–Packard (Palo Alto, CA) 1090 high performance liquid chromatograph (HPLC) equipped with an autoinjection 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, Deerfield, IL), injection volume was 20 µL, and the wavelength of the detector was set at 220 nm. Mobile phase was a mixture of acetonitrile and water that was acidified with 0.1% phosphoric acid. The ratio of acetonitrile and water (v/v) was 70:30 for napropamide and 40:60 for picloram. The flow rate was maintained at 1.0 mL min-1. Under these conditions, the retention times were 5.83 min for napropamide and 4.86 min for picloram.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Partition of Polymers
Knowing the partition of polymers between soil matrix and solution phase is necessary to understand their influence on herbicide sorption. The Kd values for the 10 polymers at different salt levels are listed in Table 1 . The results showed that charge characteristic was the primary factor controlling the partition of the polymers. The nonionic and cationic polymers, either PAMs or guars, had large Kd values at all salt levels, indicating that almost all of the polymers (>98%) were adsorbed into the soil matrix at equilibrium. However, for anionic polymers, their partitions varied with different salt levels in the solution, since their Kd values increased as salt concentrations increased (Table 1). When the salt level was low (in DI water, salt-leached soil), their Kd values were <1, indicating that the majority (>68%) of anionic PAMs (21J and 40J) and anionic guar (T-4246) remained in solution. However, when they were in 0.005 M CaCl2 solution, >95% anionic PAMs (21J and 40J) and >50% anionic guar (T-4246) were adsorbed into the soil.


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  		Table 1. One-point sorption distribution coefficient (Kd) values of polymers on the Arlington soil at varying salt levels.

 
In the normal soil pH range of 5 to 9, the surface of soil materials is predominately negatively charged. Therefore, anionic polymer sorption must overcome electrostatic repulsion. The presence of salts in the solution reduces the repulsion by cation masking, thus increasing the sorption. Cation masking also leads to a reduction in the polymer coil size (Muller et al., 1979), which allows more polymer molecules to be sorbed to the soil particle surface. The fact that highly charged PAM 40J had lower Kd value than intermediately charged PAM 21J (at the same salt level) suggests that screening the charge of anionic polymers and negative soil surface by salts is critical for polymer sorption.

Interaction between Herbicides and Polymers
Dialysis technique proved to be an effective tool for verifying the association between herbicide and humic substances (Carter and Suffet, 1982; Lee and Farmer, 1989). We used this technique to investigate the interaction between herbicides and polymers. The interaction ratios (Ri) of the two herbicides with each of the 10 polymers are presented in Table 2 . For the nonionic napropamide, no apparent difference between the inside and outside concentrations was observed (Ri values are nearly zero), suggesting that no stable bonding occurred between napropamide and the 10 polymers. For the anionic picloram, Ri values varied with polymer types and salt concentrations in the solution. At the low salt level (in DI water), Ri values were negative for the anionic polymers, nearly zero for the nonionic polymers, and positive for the cationic polymers (Table 2), indicating that picloram had a stable associative interaction with the cationic polymers and a repulsive interaction with the anionic polymers. These interactions increased as the polymer charge increased and decreased as the salt level increased. For the polymers with the same molecular structures (the PAM series or the guar series), the highly charged polymers apparently had higher Ri values (absolute value) than the low-charge polymers. For each polymer, Ri values decreased quickly with the increase in salt levels. In solutions with the medium salt level (in 0.001 M CaCl2 + 0.0005 M NaCl solution), all the Ri values were greatly reduced. In solutions with the high salt level (in 0.005 M CaCl2 solution), almost no interactions were observed, which was reflected by the near-zero Ri values (Table 2).


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  		Table 2. Interaction ratio (Ri) of two herbicides with different polymers.

 
Napropamide and some other pesticides have been shown to have the ability to form stable complexes with natural dissolved organic matter (Lee et al., 1989). It was speculated that since both the polymers and the herbicide molecules contain –CO–, –NH2, and –OH groups, stable complexes may also exist through hydrogen bonding, Van Der Waal's interaction, or other mechanisms, which lead to synergism during the sorption process. Nonetheless, the results shown above exclude this possibility, since the Ri values for napropamide, and for picloram at high salt levels (when electrostatic interaction is masked), are close to zero. Our previous research also showed that no interaction occurred between the moderately charged anionic PAM polymer (21J) and atrazine, metolachlor, and 2,4-D (Lu et al., 2002b).

However, when the salt level was low, electrostatic interactions between anionic herbicide and ionic polymers were obvious. The electrostatic attraction between high positively charged PAMs (Percol 763, 757) and picloram resulted in a concentration several times higher inside the dialysis tube than outside the tube. Based on these phenomena, it is safe to conclude that electrostatic attraction and repulsion are the primary interaction mechanisms between ionic polymers and anionic picloram.

Polymer Influence on Herbicide Sorption
The one-point distribution coefficient Kd values of the two herbicides are presented in Table 3 . For the nonionic napropamide, the Kd values in the presence of polymers showed no significant difference compared with that of the control, suggesting that polymers have no essential influence on its sorption. For anionic picloram, the polymer influence on its sorption varied with their charge characteristics and salt levels in the solution. A close look at the data in Table 3 reveals that: (i) in the absence of polymers, picloram sorption increased as salt concentration increased. The same phenomenon was observed in previous reports (Farmer and Aochi, 1974), which were attributed to the change in surface acidity of soil slurry with increasing ionic strength; (ii) the nonionic polymers (PAM 2J and Percol 333) had almost no or a slightly negative influence on picloram sorption at all salt levels; and (iii) the ionic polymers, either anionic or cationic, increased picloram sorption by the soil when the salt level became low and decreased picloram sorption by the soil when the salt level was high. However, the magnitude of the salt influence on picloram sorption in the presence of anionic polymers was small.


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  		Table 3. One-point sorption distribution coefficient (Kd) values of two herbicides on Arlington soil as affected by different polymers.

 
There are many possible ways in which polymers can influence herbicide sorption. Compared with herbicides, polymers can be adsorbed by the soil more easily and have much higher affinity to clay minerals or other soil materials due to the mechanisms of multisegment sorption of their long chains (Theng, 1982; Lu et al., 2002a). Polymer sorption on the soil matrix may: (i) affect ionic herbicide sorption by increasing (anionic polymers) or decreasing (cationic polymers) the negative charge of soil particles; (ii) decrease herbicide sorption by occupying some sorption sites making them inaccessible to the herbicide; and (iii) increase herbicide sorption by enhancing hydrophobic partitioning and other bonding interactions (such as Van Der Waal's interaction or electron donor–acceptor process) and by creating additional sorption sites for some herbicides, as presorbed polymers may be viewed as a means of increasing soil organic matter content. The net influence of polymers on herbicide sorption will depend on the soil type and the herbicide and polymer properties.

The polymer influence on herbicide sorption can be explained by the combination of electrostatic interactions and competition for sorption sites. When there are no measurable interactions occurring between herbicides and polymers, such as in the case of nonionic polymers and herbicides, ionic polymers and nonionic herbicides, and ionic polymers and ionic herbicides under high salt-level conditions, polymers have no or a slightly negative influence on the herbicide sorption due to competition for sorption sites. The small magnitude of polymer influence on herbicide sorption was attributed to the steric hindrance of large polymer molecules, which nullifies their influence on herbicide sorption on interior sorption sites of the soil matrix (Lu et al., 2002b).

When the electrostatic interaction between herbicides and polymers is not negligible (the salt level in the solution is low), polymers will exert an influence on herbicide sorption. Since the electrostatic interaction can either be attractive or repulsive, different impact mechanisms of PAMs on herbicide sorption can be expected. With regard to anionic picloram and cationic polymers, electrostatic attraction between sorbed polymers (>98% of cationic polymers were adsorbed to soil) can enhance picloram sorption. When this enhancement exceeds the negative impact from sorption site competition, a net positive influence of polymer on picloram sorption occurs (Table 3). For anionic picloram and anionic polymers, electrostatic repulsion between the herbicide and the polymers in the solution phase (the majority of anionic polymers remained in solution when the salt level was low) also facilitates picloram sorption to the soil. As the salt concentration increases, electrostatic interactions are gradually masked, and the enhancement by the presence of ionic polymers decreases. Consequently, all the Kd values tend to be slightly lower or close to the control (Table 3).

Inasmuch as salts are very common in many irrigated soils and in irrigation waters, the enhancement of anionic herbicide sorption, which takes place only at low salt levels, by ionic polymers will seldom occur. In practical conditions, it is more likely that polymers have no or a slightly negative influence on herbicide sorption, which may facilitate their movement. However, results from the batch experiments only examine the impact of polymers under laboratory equilibrium conditions. Polymers included in the formulation of pesticides may physically entrap the pesticide molecules, which lowers their leaching after application (Jain and Singh, 1991; Hall et al., 1998). Moreover, the application of polymers in irrigation waters can greatly reduce field runoff losses of pesticides (Agassi et al., 1995; Lentz et al., 1998). These benefits can negate the slight negative polymer influence on pesticide sorption.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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