Reducing Soil Erosion Losses with Small Applications of Biopolymers

Chapter 16

Reducing Soil Erosion Losses with Small Applications of Biopolymers William J. Orts and Gregory M. Glenn

Downloaded by UNIV OF ROCHESTER on August 28, 2013 | http://pubs.acs.org Publication Date: March 25, 1999 | doi: 10.1021/bk-1999-0723.ch016

Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 800 Buchanan Street, Albany, CA 94710

High molecular weight, synthetic polyacrylamides (PAM) are used increasingly in the field to prevent erosion because, as explored in this study, they are relatively large, water soluble polymers which flocculate readily with soil due to charge affinity or Van der Waals attraction. A lab-scale erosion test was established to screen biopolymer solutions for a similar efficacy in reducing shear-induced erosion. In lab-scale furrows, chitosan, starch xanthate, and acid-hydrolyzed cellulose microfibrils proved viable in reducing suspended solids in the run-off waterfromtest soil. For all of the polymers tested, erosion of clay-rich soils is reduced by increasing the concentration of exchangeable calcium. Optimization of biopolymer properties to improve their economic competitiveness in this application is discussed.

Soil erosion is a major problem threatening agricultural production and contaminating surface waters with overruns of pesticides and fertilizers. The magnitude of soil erosion is especially extensive in arid areas where soils lack the structure to withstand the shearing action of running water. For example, in the arid parts of the Pacific Northwest (Washington, Oregon, and Idaho), approximately 1.5 million hectares are surface irrigated, with erosion losses of 5 to 50 tons of soil per ha per year (7). The soil in this region is often derivedfromlava and ash and lacks polysaccharide stabilizing materials. Run-off waterfromthese fields contributes to the build-up of silt in rivers, One effective tool for reducing erosion during irrigation is the addition of small quantities of polyacrylamide (PAM) to the in-flowing water (7-5). Lentz et al (7) added 5-20 ppm of (anionic) PAM in the first 1-2 hours of the furrow irrigation of highly erodible soil and reduced sediment losses by up to 97% compared to untreated soil. This represented an ideological breakthrough in the use of soil conditioners. By adding the conditioner to the water, only the soil at the surface is treated, improving the

U.S. government work. Published 1999 American Chemical Society

In Biopolymers; Imam, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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236 cohesiveness in the -1mm thick layer at the soil surface. Thus, only small quantities of polymer are required compared to the traditional practice of adding soil conditioners to "all" of the soil in the cultivation layer. Previously, hundreds and even thousands of pounds of soil conditioner per hectare were required to effectively control erosion losses. Although the benefits of PAM are clearly seen by farmers, there are some considerations associated with this synthetic polymer which merit further review. One concern is that the long-term environmental impact of PAM is not known. PAM is a synthetic polymer that was not designed to biodegrade. It is very stable in soilexposure environments, with "slow" deamination reported as its primary route to biodegradation (6, 7). Although PAM has been used in waste treatment facilities for many years, little is known about its long-term environmental effect when used in an agricultural environment. A second consideration is that acrylamide, the monomer used to synthesize PAM, is toxiC., and any appreciable monomer impurity in the product polymer must be avoided. This problem has been suitably addressed by suppliers who provide PAM with only minute traces of monomer (#). It is imperative that producers continue to maintain a high standard of PAM in an economic environment of rapid expansion. Already, more than 700, 000 acres are treated with PAM per year (9), with that number increasing by hundreds of thousands of acres per year. Third, at its relatively high price of $3.50 - $4.50/lb, treatment for one year costs between $15 and $40 per acre. Any reduction in this price would be a clear advantage to farmers. Finally, it is not clear that PAM works equally effectively, or even at all, on certain soil types. Wallace and Wallace (5) showed that soils with specific charge/pH profiles require a different charge balance in solution than that provided by commercial PAM alone. Having an "arsenal" of polymers with an array of soil-stabilizing mechanisms would provide farmers greater flexibility in controlling erosion. In this report, we will discuss some of the functional attributes of PAM that make it an effective erosion control agent. Then, we will use this information to explore other polymers that may also be utilized in this role. Since PAM is a synthetic polymer that was not designed to achieve both biodegradability and functional performance, we will focus on biodegradable biopolymers, with long-term environmental impacts that may be better understood. There is a wide array of potentially suitable biopolymers with the primary functional attributes of PAM. They disperse readily in water, have a high molecular weight, and carry a charge. In this report, we use datafroma lab-scale soil furrow test to screen several biopolymers for their efficacy in controlling soil erosion. Materials and Methods Polyacrylamides. Polymers from a variety of sources were screened for their efficacy in controlling erosion. Polyacrylamide, PAM, samples were either linear homopolymers or linear acrylic acid copolymers, with varying molecular weights,


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CT, were trademarked Magnifloc with product codes 835A, 836A, 837A, 846A, 905N, specially provided for this project (10). Samples kindly provided by Allied Colloids, Several PAM samples ranging in molecular weight from 200,000 to 15 million were purchasedfromAldrich. All molecular weight and charge data were determined by the manufacturers.

Biopolymers. Chitosan samples were provided by Vanson, Inc., Redmond, WA, and put into solution at high concentration by the addition of weak acid solutions. After dilution of these stock solutions, the pH was adjusted back to 7.0. Most starch and cellulose derivatives were produced "in-house" using Sigma or Aldrich reagent grade chemicals. Calcium was added to test solutions by the addition of Aldrich reagent-grade calcium nitrate stock solutions. Acid hydrolyzed cellulose microfibril suspensions were formedfromcotton fiber (Whatman filter paper #4) following a procedure outlined by Revol et al. (11). Cellulose fibers were milled in a Wiley Mill to pass through a 40 mesh screen, added at 8% concentrations to -60% sulfuric acid at 60°C., and stirred for 30 minutes. The reaction was stopped by adding excess water, and the samples were centrifuged and washed repeatedly (at least three times) until clean of salts and acid. The chemically bound sulfur from this recipe, sulfonation, or surface charge, of about 10% of the surface anhydroglucose units or roughly 0.2 sulfate groups per nm . 2

Starch xanthate was produced following the procedure of Menefee and Hautala (12) using Midsol 50 wheat starch purchased from Midwest Grains. Starch was swollen by soaking in -20% (w/w) aqueous NaOH. After decanting off excess NaOH, mass. After several hours of stirring, xanthates were diluted to a 2% solution (based on polysaccharide content), which presumably stopped the reaction and facilitated further dilution. The degree of substitution, ds, was estimated using standard titration methods for sulfur content. The shelf-life of xanthate is generally limited to several days, so samples were tested within 24 hours after production.

Lab-Scale Erosion Test. Lab-scale rills or furrows were created using a modified, Erosion Research Laboratory (75). Except where noted, the soil tested was a Zacharias gravelly clay loam soil obtained from Patterson, CA, a northern California farming community 90 miles south of Sacramento. This soil was chosen because it is typical of the northern Central Valley and because PAM has been particularly effective in controlling its erosion. Soils were dried, sieved and re-moisturized to 18% (w/w) water contents and then formed into "standard" furrows which were roughly 1/100 the size of a full furrow. That is, 1500 g of moist soil was packed flat into a 2.5 x 2.5 cm well cut into a lm long bar. A furrow with dimensions 0.63 x 0.63 cm was pressed th


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lengthwise down the center of this soil rill, to create a miniature furrow. The furrow was set at an angle of 5° and test solutions were pumped down the furrow at standard flow rates, providing a shear rate profile similar to that achieved under field conditions. Water was collected at the lower end of the furrow and tested for solids contents, as determined by UV-Vis absorption. By comparing turbidity measurements (using a Shimadzu UV1601 Spectrophotometer) with those from a set of standard soil dispersions, the relative suspended solids of the water at the end of the furrow was determined. The "cleanliness" of this run-off water correlated to the efficacy of different polymer solutions in controlling erosion. It should be noted that the effectiveness of PAM varies with type of soil and the charge balance of the water. Results and Discussion Although we have screened a wide array of polymers for their ability to control erosion when added to irrigation water, this report will focus on several of the more promising biopolymer solutions. The effectiveness of any of these polymers depends on a complex interaction of (i) the soil, which is often, but not necessarily, charged, (ii) the polymer, which is often large and charged, and (iii) the water, which contains a wide array of ions. As will be discussed, the amount of available calcium in the water has a significant impact on results. Polyacrylamide, P A M . As noted, PAM has already been proven effective in controlling irrigation-induced erosion in field applications (7-5). PAM is explored in this study for two reasons; to verify the effectiveness of the furrow test to rapidly screen polymer solutions, and to highlight some of the properties of PAM that are critical for its success. Understanding the properties of PAM will help to establish biopolymers that can mimic its erosion control benefits. Several characteristics of PAM determine how it interacts with soil; its size, its affinity to minerals, and its conformation in water. Figures la and lb outline the effect of size and charge on the efficacy of PAM in our experimental furrow test. As shown in Figure 1 a, the resulting run-off water is cleaner as a function of increasing molecular weight, especially with molecular weights over 200, 000. The lab-furrow results are compared with field test datafromIdaho reported by Lentz et al (14), which show a similar trend. Considering that the error in our data is -3-4% of the control, it is not clear that there is significant improvement for molecular weights over 6 million. However, the field testsfromLentz et al (14) and information from CyteC., a PAM supplier, suggest that results are best for molecular weights exceeding 10 million. At such high molecular weights, PAM would have a fully extended end-to-end distance of several microns and would be able to form stable floes with multiple soil particles. Formation of large, stable soil floesfromsolution is key to this process. The difference between field results and lab tests are likely attributable to the fact that the soil in the lab test has been chosen specifically because of its sensitivity to PAM treatment.


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Q -B— Lab Furrows • O - Field Tests (Lentz et al)

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Reducing Soil Erosion Losses with Small Applications of Biopolymers

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