Soil Stabiliser
Anionic polyacrylamides (PAM) have been used extensively as industrial flocculent, to accelerate separation of solids from aqueous suspensions in sewage sludge dewatering, mining, paper manufacture, clarification of refined sugar and fruit juices. In washing and lye pealing of fruits and vegetables, and in adhesives and paper in contact with food. As thickeners and suspending agents in animal feeds, in cosmetics, for paper manufacturing, for various mining and drilling applications and for various other sensitive uses. Negative impacts have not been documented
Soil stabilizing polymers were used in World War II to aid road and runway construction. In the early 1950s PAM was adapted for agriculture but material and application costs limited its use to high value crops, nursery operations, etc. By the 1980s polymer cost has decreased and in the 1990s it was identified as a highly effective erosion-preventing and infiltration-enhancing polymer, by just stabilizing soil surface structure and pore continuity. It was shown that PAM applied via infiltrating water is irreversibly adsorbed in the top few millimeters of soil once dry.
PAM was first sold in the United States 1995. By 1999 about 400,000 ha of farmland in the U.S. were PAM-treated to reduce irrigation-induced erosion and to enhance infiltration.
The PAMs used are copolymers of about 150,000 repeating units per molecule, and one sodium formate (-COONa) functional group to about every five amide units. The sodium cation dissociates in water to give a negative resulting in a negative charge density of 18%. Such Coulombic forces attract soil particles to PAM and brought about particle cohesion. Minute amounts of Ca++ in the water shrink the electrical double layer surrounding soil particles and bridge the anionic surfaces of soil particles and PAM molecules, enabling flocculation. Calcium has a double charge and small hydrated radius which favors flocculation. So detached soil particles are quickly flocculated by PAM, settling them out of the transport stream. (Sodium, on the other hand, has a large hydrated radius which generally prevents ion bridging, generally leading to dispersion rather than flocculation of solids.) PAM-treated soil forms stabilized aggregates and has better pore continuity through the surface seals. Scanning Electron Micrography showed that PAM-treated soil seals have semi-continuous net- or web-like coating of polymer on, and bridging between, mineral particles. Thin (about 1 �m), porous, surface soil veneers of protective polyacrylamide coatings account for soil stabilization against furrow irrigation erosion and improved infiltration rates.
New formulations of PAM with aluminium sulfate or calcium oxide added, can now absorb nutrients, like phosphorus, and bacteria in runoff water in the soil. If animal manures are used as fertilisers it removed an impressive 99% total and faecal coliform bacteria and faecal streptococci from the runoff water. In waterways these nutrients become pollutants leading to algal blooms.
The polymer industry already knows how to produce PAM and other polymers from vegetable oils, for instance. New research has begun investigating new polymers synthesized from organic byproducts of crop agriculture and shell fish food processing which may supplement PAM for certain uses where enhanced biodegradability is needed or where bio-based chemistry is perceived to be an environmental benefit
APPLICATION
The movement of soil from raised beds, as used to grow crops such as potatoes, and the associated slumping and degradation of beds is often a serious problem. PAM reduced soil loss by 85 to 95% for the silt loam and by 70 to 80 % for silt clay. PAM eliminated an average 94% (80-99% range) of sediment loss in field runoff from furrow irrigation, with 15-50% relative infiltration increases compared to untreated controls on medium to fine textured soils. In sandy soils infiltration is often unchanged by PAM or can even be slightly reduced.
The PAMs designated for this use are regarded as "water soluble" or "linear" or "non-crosslinked" PAMs. Crosslinked" or "super water-absorbent" PAMs should not be use in furrow irrigation. It is also important to emphasize the need to use anionic PAMs in these applications. The charge density may vary from 8-30 %; a value of 18% is typical. Molecular weights of 12-15 mg/mole are optimal.
Commercial anionic moderate molecular weight PAM purchased as dry "granular," "bead" or "powder" usually contain 80% or higher active ingredient (ai). PAMs come formulated for use in four major forms, dry bead or "powder" (>80 % ai), predissolved aqueous concentrate (usually around 3% ai), compressed blocks or cubes (> 80 % ai) for suspension in flowing ditches or furrow streams, oil-emulsified concentrates (usually 30% ai). The most commonly used product is fine granular PAM. The second most common product formulations are concentrated liquid emulsions of PAM and mineral spirits. These also include "inverse emulsions" that contain a surfactant to help disperse the PAM when mixed with water. Emulsions are more commonly used with sprinkler PAM application than in furrow irrigation.
When applying PAM to furrows the soil must be dry and a 10 ppm water solution is recommended. Damp soil (water films on aggregates) prevents intimate adsorption of PAM under these conditions. Once coated it greatly reduced infiltration of the PAM-treated water that normally carries PAM to surface aggregates. PAM flowing down the furrow only a few seconds later will have the ability to reduce erosion.
In furrow irrigation PAM treats only about 25% of the field surface area to a few millimeters depth, requiring only 1-2 kg ha-1 of PAM per irrigation. Farmers typically use 3 to 5 kg ha-1 in a season depending on field conditions and crop (thus, number of cultivations and irrigations). Typical seasonal application totals vary from 3 to 7 kg per hectare. PAM's ability to increase lateral spread of water during infiltration is useful for early season water conservation. Only small amounts of water are needed to germinate seed or sustain small seedlings shortly after planting. Water conservation is accomplished by not needing to completely fill the soil profile because wetting patterns of PAM-treated furrows spread further laterally for a given volume of water applied. Research has shown no adverse effects on soil microbial populations.
The annual cost is about $15 to $35 an acre. Since it can also forestall unwanted spread of certain nutrients and microbes from fields and feedlots these new-generation chemicals are 10 times more effective than earlier compounds at only a hundredth the cost. The costs are partially or entirely retrieved by savings in erosion-related field operations, improved infiltration, water conservation, or crop responses. An ounce of PAM is reported to halt about 1000 pounds of furrow irrigation.
ENVIRONMENT IMPACT
The PAMs registered for use in the U.S. should contain no more than 0.05% free acrylamide monomer (AMD) by weight (in Europe, no more than 0.025%). AMD is a neurotoxin, however it is easily metabolized by microorganisms in soil and biologically active waters, with a half-life in tens of hours.
Neutral PAMs and especially cationic PAMs have been shown to have LC50s to aquatic organisms. Cationics are attracted to the haemoglobin in fish gills. Suffocation occurs when fish are placed in otherwise clean waters that contain low levels of cationic PAM.
In soil, PAM degrades at rates of at least 10% per year as a result of physical, chemical, biological and photochemical processes and reactions. Because PAM is highly susceptible to UV degradation, its breakdown rate when applied at the soil surface for erosion control may be faster than the 10% per year reported rate, which was for biological degradation of PAM mixed into a large soil volume.
The average PAM concentration of waters leaving a field (following the NRCS PAM application standard of 1 lb per acre) from a 24 hr irrigation is about 0.1 ppm, which is actually below the current detectability limit of PAM in natural waters. The runoff PAM only traveled 100 to 500 meters in waste ditches before being completely adsorbed on sediments in the flow or onto ditch surfaces. Where over 1,600 ha were irrigated using PAM-treated water for a two-week period. Waste water from the fields and the drain was analyzed for P, sediment, and PAM. PAM was detected in drain water samples only twice (< 0.8 kg ML-1) during monitoring. Lentz et al. (1996) developed a sensitive flocculation assay to determine PAM concentrations in surface waters at concentration as low as 0.25 ppm.
MacWilliams, D.C. (Acrylamides. Encyclopedia of chemical technology, 3rd. Ed., vol 1. Wiley, New York, pp. 298-311. 1978.) stated that breakdown of polyacrylamide to release acrylamide is thermodynamically impossible.
Prolong exposure to anionic PAMs can result in skin irritation and inflammation of mucus membranes. So care must be taken to prevent PAM dust in the air. Another caution is that PAM spills is very slippery when wet. PAM spills should be thoroughly cleaned with a dry absorbent and removed before attempting to wash down with water.
Reference:
USDA-Agricultural Research Service,
Northwest Irrigation and Soils Research Laboratory, 3793N-3600E Kimberly, Idaho 83341, USA
