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Department of Environmental Sciences, Univ. of California, Riverside, CA 92521
* Corresponding author (laowu{at}mail.ucr.edu)
Received for publication April 26, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: OM, organic matter • PAM, polyacrylamide
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Research conducted by Nadler et al. (1994) showed that tritium-labeled PAM applied on the surface soil did not move downward in a clay loam soil and only slightly penetrated through the original depth of application in a sandy loam soil after 10 mo and a 72-cm water application. This was expected since no or very low desorption of PAM occurs after its sorption onto the soil (Nadler et al., 1992). Due to its high sorptive ability on soil materials (Malik and Letey, 1991) and low mobility (Malik et al., 1991), PAM is believed to penetrate only a few millimeters below the soil surface in furrow irrigation (Sojka and Lentz, 1996). In contrast, Shaviv et al. (1987b) estimated from the size change of water-stable aggregates that negatively charged polymers (molecular weight < 75 000) could move in the soil to a depth similar to the wetting front depth when they were applied with irrigation water.
Light-scattering measurements indicated that the gyration radius of a linear anionic PAM (molecular weight of 4 Mg mol-1 and 31% NH2 group substituted by OH group) in water is about 90 to 130 nm, depending on the salt concentration in the solution (Muller et al., 1979). Although the PAM used in soil conservation technology usually has a higher molecular weight (10–15 Mg mol-1), its gyration radius is still several orders of magnitude smaller than most soil micropores. When predissolved in irrigation water, it is likely that PAM can move to some depth in soil with water infiltration. Moreover, PAM has a relatively slow sorption kinetics (Lu et al., 2002a), which can facilitate its movement during the time of water penetration. To fully understand PAM penetrability in soil, reliable quantitative information of PAM distribution with soil depth is greatly needed.
However, difficulties in the quantification of PAM on soil hinder the study of PAM movement. All the methods available in earlier literature are developed for determination of PAM in aqueous solutions (Lu and Wu, 2002). Since PAM is highly water soluble and irreversibly binds to soil by a multisegment sorption mechanism (Theng, 1982), it is almost impossible to extract PAM from the soil. Any analytical methodology regarding soil-sorbed PAM quantification should be able to detect PAM directly on the soil. Recently, the aqueous solution–based N-bromination method (Lu and Wu, 2001) was modified to quantify substrate-borne PAM (Lu and Wu, 2002), which makes it possible to measure the content of PAM retained in soil.
The objectives of this research were to (i) determine the distribution of PAM in soil columns after surface application; (ii) investigate the factors (salt level, soil texture, initial soil water content, and water application methods) affecting PAM distribution; and (iii) investigate the mechanisms controlling PAM movement in soil. The results will provide useful information for improvement of PAM application methodology and for better understanding of PAM movement behavior, leaching ability, and its potential effect on the environment.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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re, 1979; Velde, 1992). Since the loss of hygroscopic water and cation-bound water is reversible, the chemical properties of OM-removed soil are supposed to be similar to the inorganic component of natural soil. Moreover, unlike the sorption of small organic molecules (such as some pesticides), which depends heavily on soil OM content, PAM sorption on soil is much less dependent on OM content. Sorption of PAM on partially OM-removed soils is only slightly higher (5–30%) than on the natural soils (Lu et al., 2002a). However, the soil aggregate stabilized by OM will be destroyed during the combustion.
Column Experiment
A transparent plastic column with a 5.2-cm diameter was split in the middle. Silicone sealant was applied to the edges of the two halves to avoid leaking. Then the two halves were reassembled using clear plastic tape. Organic matter–removed soils were uniformly packed to a height of 60 cm by gentle tapping with a rubber hammer. Using this method, a bulk density of 1.68 g cm-3 was achieved for the Hanford sandy loam and 1.33 g cm-3 for the Imperial silty clay. A solution of PAM (74.7 mL, equal to an 8-cm application) was applied on the top of the columns, either by ponded or drip application. Forty-eight hours later, the column was opened and the soil was sectioned into 1-cm-increment samples. To obtain a representative sample, each sample from the same depth was thoroughly mixed and three subsamples (1.0–2.0 g dry weight equivalent each) were taken for PAM analysis. The rest was used for 105°C oven-dry moisture measurement.
In the water-ponded application experiments, PAM solution was applied at the soil surface with a constant head of about 2 cm. In the drip-application experiments, PAM solution was applied to the top of the column as droplets (through a rubber tube with an inner diameter of 2.1 mm) without allowing water accumulation (maintaining an unsaturated condition) at the column surface. The drip application rate was 1 cm h-1. A fast filter paper (No. 4; Whatman, Maidstone, UK), presoaked with the applied PAM solution, was placed on the top of the soil column to evenly distribute water and to avoid possible disturbance when water dropped onto the soil surface. In each water application method, PAM solution with a concentration ranging from 10 to 40 mg L-1 prepared in deionized water, 0.001 M CaCl2, and 0.005 M CaCl2 were investigated.
To investigate the effects of initial water content on PAM distribution, a known amount of deionized water was added to soil and the soil was mixed thoroughly before being packed into the columns. After packing, the columns were left two days for water redistribution before applying PAM solutions.
Each treatment was repeated three times. The standard deviations of the measured PAM contents at the same soil depth between the three replicates were within 10% and their mean was reported. The total mass of PAM found in the soil was 91 to 98% of the amounts applied in all columns.
Sorption Experiment
Polyacrylamide sorption isotherms on two OM-removed soil samples were determined by the batch equilibrium method. Soil samples were added to PAM solution with concentrations ranging from 2 to 40 mg L-1 and shaken 36 h for sorption equilibration. The solution to soil ratios were kept in the range from 10 to 100 under different salt conditions to get an appropriate final PAM concentration in the supernatants. Experimental details can be found in Lu et al. (2002a). Sorption under different salt levels (deionized water, 0.001 M CaCl2, and 0.005 M CaCl2) was investigated.
Determination of Soil-Sorbed Polyacrylamide
The modified N-bromination method was used for the determination of soil-sorbed PAM. Deionized water was added to a soil sample containing PAM to form a suspension and PAM was exposed to analytical reagents on its sorbed state. The quantity of PAM was converted to the concentration of a reaction intermediate in aqueous solution that was then measured by spectrophotometry. A detailed analytical procedure was given by Lu and Wu (2002). Calibration curves for soil-sorbed PAM were obtained by measuring a series of samples from the same soil of known PAM contents. An individual calibration curve was established for each soil. The method has a lower detection limit of about 2 µg PAM in the Hanford sandy loam and about 4 µg PAM in the Imperial silty clay, and the optimum range of PAM content in an assay sample is 5 to 80 µg.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Second, a high amount of the applied PAM was retained in the top few centimeters of the soil. Typically, the mass of PAM retained in the top 0 to 2 cm of the soil was about 23 to 95% of the total applied amount (with ponded application). Polyacrylamide content (per soil mass) in the top 0 to 2 cm of the soil was 2 to 10 times higher than that in the following 2 to 10 cm of the soil. The content of PAM decreased dramatically with soil depth in the top 0 to 4 cm of the soil, then it decreased slowly in the next layer of 4 to 20 cm of the soil. This phenomenon became more obvious when the salt level in the applied PAM solution was low, such as in the cases of deionized water and 0.001 M CaCl2. The retention of PAM in the top few centimeters of the soil remained high even when the concentration of the applied PAM solution was low (Fig. 2) .
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Polyacrylamide Distribution and Sorption
Sorption and desorption is one of the most important factors controlling chemical movement in the soil. Sorption retards its penetration. As its sorptive affinity on soil increases, the chemical's mobility decreases. Compared with pesticides or other small molecular agrochemicals, PAM has very high sorptive affinity on soils (Malik and Letey, 1991) and once sorbed into the soil, no or very little desorption occurs (Nadler et al., 1992), which causes the high retardation of PAM movement in soil. Polyacrylamide sorption isotherms on the two OM-removed soils under various salt levels (Fig. 3)
can be well described by the Langmuir equation (Lu et al., 2002a). The fitted Langmuir saturation sorption amounts of PAM on the OM-removed Hanford sandy loam were 0.098, 0.193, and 0.358 g kg-1, respectively, and on the OM-removed Imperial silty clay were 0.258, 0.549, and 1.981 g kg-1, respectively, for the salt levels of deionized water, 0.001 M CaCl2, and 0.005 M CaCl2. This explains why the PAM penetration depth observed in the same soil decreases as the salt level increases. The saturation sorption amounts of PAM on soils were highly dependent on salt levels and soil texture (Lu et al., 2002a), and so was the penetrability of PAM into the soil. Polyacrylamide sorption onto the soil is one of the critical factors controlling PAM movement in soil.
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Polyacrylamide Distribution and Water Application Methods
Polyacrylamide distribution with soil depth in the OM-removed Hanford sandy loam under drip application is presented in Fig. 4 . The shape of PAM distribution curves is similar to that of ponded applicaton (Fig. 1a). Higher amounts of PAM were retained in the top few centimeters of the soil than in the soil below. The mass of PAM retained in the top 0 to 2 cm of the soil was about 16 to 66% of the total applied amount. However, the comparative magnitude of PAM retention in the top few centimeters was smaller and the PAM penetrated slightly deeper under drip application than under ponded application. More PAM was retained on the surface soil under ponded application. These results indicated that under the column conditions in this study, if deeper PAM conditioning is desired, PAM-containing water should be applied at a low rate by drip or trickle irrigation to avoid ponding. On the other hand, if PAM is aimed to mainly stabilize the surface soil, ponded irrigation (such as furrow and flood irrigation) is preferred. Nevertheless, if the soil has macropores or cracks, which can facilitate the occurrence of preferential flows, the results may be different.
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Polyacrylamide Distribution and Initial Soil Water Content
The effect of initial soil water content on distribution of PAM with depth is presented in Fig. 5
. As the initial soil water content increased, the amount of PAM retained in the top few centimeters of the soil decreased and the penetrability of PAM increased. With the 8-cm ponded application and a salt level of 0.001 M CaCl2, the PAM penetration front was at 16 cm in dry soil and increased to 19 and 24 cm, respectively, with initial soil water contents of 4 and 8% (Fig. 5).
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Mechanisms Controlling Polyacrylamide Distribution
The PAM retained in the soil during the water penetration process includes two parts, the sorbed PAM retained on the soil matrix and the solution PAM remaining in soil pores after the flow passes. The solution-phase PAM also becomes sorbed onto the soil once the soil dries up (Nadler et al., 1992). The retention process of PAM is governed by many physical and chemical factors. Physically, the delivery and diffusion process of PAM molecules to the soil matrix is controlled by PAM molecular characteristics, viscosity of PAM solution, and the flow pattern, which is affected by soil texture and structure, initial water content, irrigation method, and other soil chemical and physical conditions. Since PAM is entrained in infiltration water, the pathway of convective mass flow determines the possible distribution of PAM in the soil profile. It can be expected that deeper PAM penetration could be observed in some parts of a field than elsewhere if there exist preferential flow pathways throughout the field. Chemically, the bonding of PAM molecules to soil sorption sites is controlled by PAM sorptive affinity and sorption kinetics. Conditions favoring PAM sorption such as high salt level, fine soil texture, better solution–particle contact condition, and longer solution–particle contact time decrease PAM mobility and increase the retention rate of PAM by soil.
The PAM distribution curves with soil depth clearly display two different parts (Fig. 1 and 2, and Fig. 4 and 5). In the top few centimeters of soil, PAM behaves as a chemical with very high sorptive affinity on soil, characterized by a high retention rate and dramatic drop of PAM content with increasing soil depth. In the lower layer of the soil, PAM behaves as a chemical with lower sorptive affinity, indicated by the lower retention rate and the slow decrease of PAM content with increasing soil depth. This phenomenon implies that the conditions that affect the underlying mechanisms for controlling PAM movement in the topsoil and the lower layer of the soil are different. From the chemical point of view, there is no obvious difference between the factors affecting PAM sorption on the topsoil and on the lower layer of the soil. If the retention of PAM in soil is mainly controlled by the sorption of PAM, the retention rate of PAM in soil should decrease smoothly without an apparent change between the top few centimeters of the soil and the soil right below that. Thus, the abrupt change in PAM concentration in the profile was attributed to three physical factors that may cause the much higher retention rate of PAM in the top few centimeters of soils.
First, during the water application process, the top few centimeters of the soil had more favorable solution–soil contact condition and longer contact time with the flowing PAM solution, which allowed more PAM molecules to diffuse into the soil matrix and then become adsorbed. Such differences could be significant due to the relatively slow kinetics of PAM sorption on the soil.
Second, when the PAM solution was applied to the top of the soil column, the first few centimeters of the soil immediately became soaked with the applied solution. The amount of polymer moving into soil particles by convective mass flow could be in excess of what was needed for sorption in that time period, especially when the soil was dry.
Third, since PAM molecules move slower than water, when the flow containing PAM passed through the soil below the top few centimeters, it was already wetted by earlier water containing no PAM. The retention rate of PAM in prewetted soil is lower than in dry soil (Fig. 5).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The flow pathway delimits the possible distribution area of PAM in the soil profile. The PAM sorptive affinity and kinetics mainly determine the amount of PAM retained by soil. High sorptive affinity on soil is the main reason for the low mobility of PAM in soil. More favorable solution–soil contact condition, longer soil–solution contact time, and lower initial soil water content cause the particularly high PAM retention in the top few centimeters of the soil compared with that in the lower layers of the soil.
It should be pointed out that the conditions between the laboratory columns and the field conditions were quite different. In the field, PAM would probably penetrate deeper than observed in the laboratory columns that were uniformly packed using structureless OM-removed soil because (i) sorption of PAM on the soil with OM removed was greater than sorption on the natural soil (Lu et al., 2002a) and (ii) in natural soils with aggregates and structural heterogeneity, substantial amounts of water could pass through soil discontinuities such as desiccation cracks, gopher holes, or worm channels during water infiltration, which leads to deeper water and PAM penetration. However, the effects of salt concentration, soil texture, initial soil water content, and water application methods on PAM movement behavior should remain the same. Although the aggregates stabilized by the OM were destroyed by combustion, results from this study are still helpful in increasing our understanding of PAM movement and distribution in soil since the chemical properties of OM-removed soil remain similar to the inorganic components of natural soil.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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re. 1979. Thermal analysis, DTA, TG, DTG. p. 243–261. In H. Van Olphena and J.J. Fripiat (ed.) Data handbook for clay minerals and other non-metallic minerals. Pergamon Press, Oxford.Related articles in JEQ:
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