| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Dep. Environmental Sciences, Univ. of California, Riverside, CA 92521
* Corresponding author (laowu{at}mail.ucr.edu)
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Abbreviations: CEC, cation-exchange capacity • DI, deionized • PAM, polyacrylamide • OM, organic matter • TDS, total dissoved salts
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
10–20%) PAMs are the most effective types in soil application (Barvenik, 1994; Anonymous, 1995). Addition of PAM to soil will affect soil dispersion, flocculation, and aggregation (Ben-Hur et al., 1992). Knowledge of the sorptive behavior of PAM is useful in predicting appropriate dose of application, depth of effective treatment, its mobility in soil, and changes in soil physical conditions. With good knowledge of polymer–soil interactions, the optimal amount of polymer application can be potentially prescribed.
Polyacrylamide is one of the most widely used polymers. Its sorption by sand, silica, alumina, clay minerals, latex, cellulose, and other materials has been the topic of previous publications. Greenland (1972) and Theng (1982) presented comprehensive reviews on the sorption of polymers in clay suspensions. Pefferkorn (1999) discussed interfacial processes involving PAM sorption. Nevertheless, little information was found in the literature regarding the sorption reaction between soil and PAM. In addition, the molecular weight of PAM used in irrigation is much higher but the concentration is much lower compared with those earlier studies. Information on PAM sorption at concentrations as low as 10 mg L-1 is sparse, possibly because of the difficulty in determining PAM concentrations in soil solutions (Lu and Wu, 2001).
Nadler and Letey (1989) and Malik and Letey (1991) determined sorption isotherms of several types of polyanion by an Arlington sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralfs) through the use of tritium labeled polymers. Their results suggested that polymer sorption by soil was mostly limited to external (outer) surface area and was considerably influenced by water quality (Aly and Letey, 1988). Nadler et al. (1992) also found that there was little desorption after the polymers were adsorbed onto soil.
Soil and water properties such as texture, clay mineralogy, OM, and concentration of dissolved salts will affect PAM sorption. Knowledge of the extent these factors will affect PAM sorption is still lacking. The objective of this study is to investigate the effects of these soil and irrigation water properties on the sorption of anionic PAM by soils.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Six soils from the western United States of America, a Linne clay loam (fine-loamy, mixed, superactive, thermic Calcic Pachic Haploxerolls), an Imperial silty clay (fine, smectitic, calcareous, hyperthermic Vertic Torrifluvents), an Imperial silt loam (mixed, calcareous, hyperthermic Typic Torrifluvents), a Palouse silt loam (fine-silty, mixed, superactive, mesic Pachic Haploxerolls), an Arlington loamy sand, and a Hanford sand (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents), were selected to obtain a wide range of texture and OM contents (Table 1). Soil samples were collected from the top 10-cm layer of the profiles, air-dried, and ground to pass through a 1-mm sieve. Organic matter content was determined by 450°C combustion method (Davies, 1974), and particle-size distribution by the hydrometer method (Gee and Bauder, 1986).
|
Sorption isotherms were determined by a standard batch equilibrium method. Clay materials or soil samples were added to PAM solution with concentrations ranging from 2 to 40 mg L-1 in a series of 25-mL glass bottles. The solution/soil or clay ratios ranged from 10 to 100 for soils, and 50 to 200 for clays under different cation conditions to get an appropriate final PAM concentration in supernatant. Preparatory experiments showed that sorption of PAM increased slightly with increasing solution/soil or clay ratio, but <10% within the above ranges.
Preliminary experiments showed that more than half of sorption occurred in the first 30 min and >85% in the first 5 h. The final equilibrium took about 15 to 22 h, depending on soil characteristics and dissolved salt concentrations in PAM solution. In this research, capped bottles were shaken for 36 h at 25 ± 2°C on a reciprocal shaker. Following that, 10 mL supernatant was centrifuged for 5 min. at 5000 x g to remove possible suspended particulates before analysis. The amount of PAM adsorbed was calculated from the difference between the initial concentration of the PAM and the concentration remaining in solution at the end of the sorption run. All experiments were duplicated.
Dry clay particles tend to coagulate when added to PAM solution, which can cause incomplete mixing between the adsorbate and adsorbent. Thus, a clay suspension (1 g L-1) instead of dry clay was added to PAM solution in this research. To achieve uniform distribution, the suspension was subjected to vigorous stirring before it was taken out and added to the PAM solution.
To evaluate the role of the OM in PAM sorption, isotherms were also carried out on the same soils after removing a portion of their OM. A range of OM contents was obtained by consecutive additions of aqueous 10% H2O2 (Palmer and Troeh, 1977). After one and two oxidation treatments, the OM contents of the Palouse silt loam was reduced from 54.5 to 33.4 and 13.2 g kg-1, and the Linne clay loam was reduced from 38.8 to 32.4 and 20.1 g kg-1, respectively. The Imperial silty clay was reduced from 24.6 to 13.5 g kg-1, and the Arlington loamy sand from 19.3 to 8.4 g kg-1 after one treatment. After oxidization, the samples were heated to 95°C in a water bath for 10 min to remove excess H2O2. They were then air-dried and ground to pass through a 1-mm sieve.
The method of Lu and Wu (2001) was used to determine PAM concentration in soil supernatants. This method was based on determination of amide groups by the N-bromination method and deduction of interferential moiety of dissolved OM by spectrophotometry. It has a detection limit of 0.2 mg L-1 and a linear range from 0.2 to 60 mg L-1 without dilution of original samples. The concentrations of cations in soil supernatants were determined by Inductively Coupled Plasma (ICP) in an Optima 3000 DV Spectrophotometer (Perkin Elmer Corporation, Norwalk, CT). The analysis wavelengths for Na+, K+, Ca2+, and Mg2+ are 589.592, 766.490, 317.933, and 285.213 nm, respectively.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
|
 | [1] |
The fitted amounts of PAM saturation sorption (As) are presented in Table 2. Compared with most pesticides and herbicides, the amount of PAM sorption was very high. The high affinity of PAM to clay minerals and soils are because of the mechanisms of multi-segment sorption of its long chain (Theng, 1982).
|
As a result, the amount of PAM sorption by fine particles was greater than by large particles (Malik and Letey, 1991). Figure 1 clearly shows that in solutions of equal salt concentration, PAM sorption was in the order of montmorillonite > kaolinite > fine sand, which agrees with the order of their specific outer-surface areas. In the presence of salts at a certain concentration range, clay minerals can adsorb PAM several times greater than soils do, and ten or several hundred times greater than the fine sand does (Table 2).
As the chemical compositions of inorganic components of soils are mainly silicate or alumino-silicate materials, it is safe to assume that per unit surface area of sand, silt, and clay fractions have similar active sites for PAM sorption. Soils with fine texture, such as Linne clay loam and Imperial silty clay, tend to adsorb more PAM than coarse-textured soils such as Arlington loamy sand and Hanford sand when the solution contains the same type and concentration of cations (Table 2).
The effect of thickness and shape of clay tactoid on PAM sorption was evaluated by adding various amounts of salts to the suspension. When PAM was prepared in DI water or when the cation was Na+ (PAM was prepared in NaCl solution), the amount of PAM sorption by montmorillonite was only slightly higher than by kaolinite. However, when Ca2+ was present, the amount of PAM sorption by montmorillonite was two to three folds of that by kaolinite, even though the salt concentration was the same (Table 2). In the presence of Ca2+, the number of plates per tactoid of montmorillonite tends to decline (Green et al., 1978), which provides a relatively large total accessible surface area. However, this does not happen with kaolinite. A PAM study conducted by Ben-Hur et al., (1992) using montmorillonite and illite, showed similar results.
Effect of Dissolved Salts on Polyacrylamide Sorption
The isotherms in Fig. 1 and 2 indicate that the sorption of anionic PAM by the same minerals and soils increased significantly as the concentrations of dissolved salts increased. For all six test soils, the amount of PAM saturation sorption in 0.01 M NaCl was 0.5 to 1.5 and in 0.005 M CaCl2 was four to nine times greater than that in DI water. Soils with high dissolved salt content, such as the Arlington loamy sand, adsorbed more PAM than the Palouse silt loam, even though the texture of the Arlington loamy sand was coarser than the Palouse silt loam when PAM solution was prepared in DI water. Clay minerals such as montmorillonite and kaolinite have high accessible surface areas and can adsorb more PAM than soils do, but the magnitude of PAM sorption was similar by both the minerals and soils in DI water (Fig. 1 and 2), because of the lower salt concentration in clay than in soil suspension. In contrast, minerals adsorbed five to ten times more PAM than soils in 0.005 M CaCl2. The results in Table 2 showed that clay minerals adsorbed relatively low amounts of PAM in DI water relative to their particle sizes (Table 2), which was attributed to lower salt concentrations in the clay suspensions than in the soil suspensions.
The nature of interactions between anionic polymer and soil material surfaces is still not well understood. Hydrogen bonding (Kohl and Taylor, 1961; Nabzar et al., 1984) and ligand exchange (Theng, 1982) are two bonding mechanisms often suggested by earlier researchers. H-bonds usually occur between the amide group of polymer and the free hydroxyl group of the adsorbent surface, which are not H-bonded with other neighboring hydroxyls (Griot and Kitchener, 1965; Pefferkorn et al., 1990). While in ligand-exchange bonding mode, the carboxylic group of the PAM enters the inner coordination layer of the edge Al to form a coordination complex. However, as both anionic PAM and surface of soil materials are negatively charged in a normal soil pH range of 5 to 9, electrostatic repulsion prevents PAM sorption through H bonding, ligand exchange, or other unknown mechanisms. As a result, the sorption process is entirely governed by the competition between polymer attractive interaction with soil surfaces and repulsive electrostatic forces, as observed in the case of hydrolyzed PAM sorption by siliceous materials (Lecourtier et al., 1990). The PAM sorption therefore increases rapidly when electrostatic repulsion is reduced by increased cation masking. Moreover, this reduction of electrostatic repulsion also favors the attraction between adsorbed and nonadsorbed PAM molecules, thereby allowing more molecules to approach the interface and to be adsorbed.
Another possible mechanism for enhanced PAM sorption by cation masking may be attributed to the change of configuration in PAM molecular structure. Light-scattering measurement showed that the average gyration radius of the hydrolyzed PAM molecule in aqueous solution tends to decrease in the presence of salts (Muller et al., 1979).
Major cations in soil solution include Na+, K+, Ca2+, and Mg2+ (Wolt, 1994). To show the effect of different valent cations on PAM sorption, the amounts of PAM saturation sorption by six soils were plotted against the concentration of monovalent cations (Na+ and K+) and divalent cations (Ca2+ and Mg2+), respectively (Fig. 3) . The concentrations of cations were measured in the supernatant after each sorption experiment with a 20 mg L-1 initial concentration of PAM. Figure 3 shows that divalent cations (Ca2+ and Mg2+) are much more effective than monovalent cations (Na+ and K+) in enhancing PAM sorption by soils at the same concentration, as indicated by the data points for all six soils being steeper in case of Ca2+ and Mg2+ than those of Na+ and K+. It was estimated from the average of slopes in Fig. 3a and 3b that the divalent cations are about 28 times more efficient in enhancing PAM sorption than the monovalent cations in the range of concentrations used in this study. This is in the same magnitude with their relative flocculation power (Na+ = 1, K+ = 1.8, Mg2+ = 27, and Ca2+ = 45; Rengasamy and Sumner, 1998). Since the concept of relative flocculation power is a comprehensive index of charge screening ability for cations, this phenomenon signified that the mechanism of PAM sorption enhancement by salts is mainly because of their charge screening ability. Another possibility for divalent cations being more effective than monovalent is that divalent cations, such as Ca2+, may act as a binding ion between the carboxylate groups of PAM chain and the anionic surface sites of soil particles (O'Gorman and Kitchener, 1974; Theng, 1982).
|
5.0 mM, the increasing amounts of PAM saturation sorption by the six test soils were 0.37, 0.45, 0.60, 0.99, 1.54, and 1.63 mg g-1 for the Hanford sand, Arlington loamy sand, Palouse silt loam, Imperial silt loam, Imperial silty clay, and Linne clay loam, respectively. Roughly, this follows the increasing order of clay and silt contents of the six soils. This result suggests that the function of charge screening of the same cations is more effective in fine soils than in sandy soils, possibly because of the higher negative charge density on the surface of fine soils than the sandy soils. The same trend was also found for the monovalent cations (Na+ and K+), except that the magnitude of increase in PAM sorption was much less compared with divalent cations. Electrostatic repulsion is strong enough to prevent any PAM sorption in some cases. Lecourtier et al. (1990) found that there exists a critical salt concentration level for highly charged particles such as silicon carbide. No sorption was observed below that level. For soils, previous studies also indicated that attractions between negatively charged soil and PAM requires a small quantity of divalent cations in water to shrink the electrical double layer and bridge the soil and PAM negative charge sites to enable flocculation (Wallace and Wallace, 1996). Our results suggest that when anionic PAM is applied to dispersive soils, which are usually highly negatively charged, a certain amount of Ca2+ in irrigation water is necessary to ensure the effectiveness of PAM application, since sorption is the prerequisite of PAM function in stabilizing soil aggregates.
Effect of Organic Matter on PAM Sorption
The amounts of PAM saturation sorption by four natural soils and their subsamples with OM lowered by H2O2 oxidization are plotted versus their OM contents in Fig. 4
. It is obvious that the sorptive affinity of the soils to PAM increased after some of the OM was removed. For example, the amount of PAM saturation sorption of the Linne clay loam increased from 0.28 to 0.32 and 0.38 mg g-1 after its OM contents were lowered from the original 38.8 to 32.4 and 20.1 g kg-1, respectively. This phenomenon was in agreement with earlier observations by Nadler and Letey (1989). Since the oxidation process did not change texture and salinity, the increase in PAM sorption was mainly because of the decrease of OM content. Hence, it was concluded that OM in soil had a negative effect on PAM sorption. This helps to explain why soils with high OM content such as the Palouse silt loam, have a relatively low PAM sorption in DI water (Table 2), even though the texture may be fine.
|
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). Soil OM plays an important role in forming soil aggregates. Removal of soil OM breaks down soil aggregates and exposes new accessible sites for PAM sorption, which definitely increases the sorption amount of PAM. In addition, in the normal soil pH range of 5 to 9, the majority of functional groups in OM carry a negative charge. Decrease in OM content reduced the electrostatic repulsion between soil particles and anionic PAM molecules and thus increased PAM sorption.
It is true that some functional groups such as –OH, –COOH, –Phenyl OH, and –NH2 might form H-bonds with –COOH and –NH2 groups in PAM molecule. Thus high OM may favor PAM sorption. However, the increase of PAM sorption after OM oxidization showed that this kind of H-bonding could not negate the above-mentioned two mechanisms, possibly because the direct sorption of PAM by OM is only a small proportion of the total PAM sorption. This is quite different from the behavior of small molecular compounds such as pesticides and herbicides. In the latter, OM may be decisive in determining the amount of sorption, since most pesticides or herbicides have hundreds or thousands times of affinity with OM than with inorganic components.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
In the normal soil pH ranged 5 to 9, sorption of the anionic PAM must overcome electrostatic repulsion as the majority of soil surfaces are negatively charged. The presence of cations in aqueous solution, either from the salinity of soils or from irrigation water, significantly increases PAM sorption. The efficiency of the cation enhancement depends mainly on their charge screening ability. For the six soils used in this study, divalent cations, such as Ca2+ and Mg2+, are about 28 times more effective than monovalent cations such as Na+ and K+ under the same concentration.
High OM contents reduce the PAM sorption, possibly because of the reduction of accessible sorptive sites since OM can cement inorganic soil components into soil aggregates and increase the electrostatic repulsion between particle surfaces and PAM molecules.
This research explored the effect of soil and water properties on PAM sorption using a batch equilibrium experiment. The isotherms of PAM sorption under the field conditions might be different, which will be the topic of our future research.
|
NOTES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Received for publication February 15, 2001.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  R. A. McLaughlin and N. Bartholomew Soil Factors Influencing Suspended Sediment Flocculation by Polyacrylamide Soil Sci. Soc. Am. J., March 12, 2007; 71(2): 537 - 544. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. A. Ajwa and T. J. Trout Polyacrylamide and Water Quality Effects on Infiltration in Sandy Loam Soils Soil Sci. Soc. Am. J., February 27, 2006; 70(2): 643 - 650. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Blanco-Canqui, C. J. Gantzer, S. H. Anderson, and A. L. Thompson Soil Berms as an Alternative to Steel Plate Borders for Runoff Plots Soil Sci. Soc. Am. J., September 1, 2004; 68(5): 1689 - 1694. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Lu, L. Wu, and J. Gan Determination of Polyacrylamide in Soil Waters by Size Exclusion Chromatography J. Environ. Qual., September 1, 2003; 32(5): 1922 - 1926. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Lu and L. Wu Polyacrylamide Distribution in Columns of Organic Matter-Removed Soils following Surface Application J. Environ. Qual., March 1, 2003; 32(2): 674 - 680. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Lu, L. Wu, J. Letey, and W. J. Farmer Anionic Polyacrylamide Effects on Soil Sorption and Desorption of Metolachlor, Atrazine, 2,4-D, and Picloram J. Environ. Qual., July 1, 2002; 31(4): 1226 - 1233. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Lu, L. Wu, J. Letey, and W. J. Farmer Picloram and Napropamide Sorption as Affected by Polymer Addition and Salt Concentration J. Environ. Qual., July 1, 2002; 31(4): 1234 - 1239. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
0.002 M NaCl; (iii) 
0.010 M NaCl; (iv) 
0.001 M CaCl2; and (v) 
0.005 M CaCl2. Each data point is the mean of two replicates and error bar is its average deviation. Solid line is fitted Langmuir sorption isotherm.
