miracle, myth…or marketing Super-absorbent water crystals

re hydrogels really so “super?” and homeowners alike, these latter While gardeners may not be compounds are the focus of this article. familiarA with the term “hydrogel,” nearly everyone who has worried about watering How do hydrogels work? Linda Chalker-Scott, PhD plants has heard about “watering crystals” In addition to their solubility, MasterGardener WSU editor or “water retention granules.” Th e ability hydrogels are also defi ned by their Extension Urban Horticulturist of these synthetic to absorb overall chemical charge: they may be and Associate Professor, water and selectively bind to other characterized by a negative (anionic), Puyallup Research and Extension Center, substances has led to their use as positive (cationic) or neutral charge. Washington State University stabilizers, water purifi ers, juice clarifi ers, Th ese charge classes are found in both Puyallup, Washington animal feed thickeners, and in the pro- linear and cross-linked PAM; the charge cessing of oil, pulp and paper, and fruits determines how they will react with and vegetables. Moreover, they are widely and solutes.

Jim Black used in cosmetics and other personal products, and have medical application in Cationic PAM hydrogels tissue augmentation (see “ up close and very personal”). should not be used in Although our main interest is in the eff ectiveness of garden-variety hydrogels, gardens and landscapes. it is important to realize how widely these compounds are used in other contexts Briefl y, clay components of soils have and to understand their positive—and a negative charge; heavy metals have a negative—impacts. positive charge, and other commonly Th ere are two broad classes of found minerals in soils and water may (PAM) hydrogels: soluble possess either a positive or a negative (linear) and insoluble (cross-linked). charge, depending on the compound in Linear PAM dissolves in water and question. Th erefore, cationic PAMs (+) has been successfully used in reducing generally bind to clay components (-) and irrigation-induced erosion in agricultural act as fl occulants (congealers); anionic fi elds(see “Hydrogel use for erosion PAMs (-) cannot directly bind to clay control”). (-) and may act as dispersants. However, Cross-linked PAM does not dissolve, anionic PAMs can bind to clay and but forms a when water is added and other negatively charged particles in the is oft en used in garden, landscape, and presence of ionic bridges, such as calcium nursery situations as a way of retaining (Ca+2). moisture. How polyacrylamide will act Insoluble PAM products are in any given situation can be hard to marketed as “superabsorbent gels” or predict, as the chemical interactions “hydrating crystals.” Instead of dissolving, between the gels, soil components, and these gels absorb water, swelling to dissolved substances are complex and many times their original size. As they occur simultaneously. Electrical charges, Polyacrylamide gels are available in dry, water is slowly released to the soil. hydration levels, van der Waals forces, many Pacifi c Northwest nurseries. Popular with the nursery industry and hydrogen bonding all modify the

AUTUMN 2007 www.MasterGardenerOnline.com • MASTERGARDENER • PAGE 35 affinity of the gel for other compounds. of cross-linked gels to enhance calcium applicable to landscapes. Indeed, it is The polyacrylamide contains ion availability, reducing the amount of under such conditions (e.g., revegetation a complex array of positively charged, sodium uptake. of quarries and mines) over the long-term negatively charged, and neutral chain According to other researchers, that PAM gels perform most poorly. segments, all with varying affinities however, PAM did not improve plant for other molecules. The stronger survival compared to control or other Long-term effectiveness the attraction between the gel and treatments, especially if performance Although synthetically produced, surrounding solutes and soil particles, was evaluated over time. In several cases, polyacrylamides are organic chemicals the greater the ability of the gel to absorb PAM-treated plants performed worse that can be degraded by both living water, create aggregates, and stabilize soil than the untreated controls. Moreover, and non-living environmental factors. structure. excessive use of PAM can lead to nutrient Exposure to ultraviolet radiation, Unfortunately, on-the-ground deficiencies; phosphate and silicon were chemical oxidizers, fertilizer salts, conditions can prevent PAM hydrogels reduced in tomato and wheat, and this mechanical abrasion, and freeze-thaw from functioning optimally. Fertilizers latter plant also suffered manganese and events will degrade the polymer, breaking and other dissolved substances can boron deficiencies when grown under it up into smaller fragments. These interfere with hydrogel water-holding high PAM concentrations. smaller fragments do not have the same capacity. Hot, dry weather conditions Why is there such high variability properties as the larger polymers, and can lead to increased degradation and among research results? I believe the cause thus the hydrogel’s water-retaining decreased effectiveness of PAM hydrogels. is both environmental and temporal. capacity and other functions are And for every success story, one can find Many of the positive results are drawn reduced and ultimately lost. Gels that a situation where hydrogels have failed to from studies that are short-term and/or are applied to soil surfaces experience function. performed under controlled conditions; these environmental stresses most for instance, one study reports on frequently and will degrade most rapidly, Hydrogel effects on plants tree survival only a few months after especially in soils with high levels of solar The documented impacts of cross- installation. As we already know, PAM ultraviolet. linked PAM hydrogels on plant survival gels lose their water-holding effectiveness Even if gels are protected from and establishment are variable. Some over time, especially when exposed environmental exposure, they will still researchers report enhanced growth of to high levels of ultraviolet, salts, and be broken down by decomposition. crop and tree species. Presumably this freeze-thaw cycles. Positive results in A number of naturally occurring enhancement is caused by improved the short-term may be perfectly valid soil microbes have been identified as soil water conditions, though in some for nursery plant production, where active decomposers of both soluble cases salt tolerance is also reported. This environmental conditions can be more and cross-linked polyacrylamide gels. latter ability may be due to the ability tightly controlled, but they are not as Decomposers include bacterial species

Polyacrylamides: up close and very personal

eople are exposed to polyacrylamides every day. More than flammation, and even cancer. The recommended treatment for Pa hundred formulations are used in cosmetics such as sun- complications arising from injectable polyacrylamide gels is full screen, shampoo, soap, lotion, and shaving cream. They are used removal of the gel and replacement with silicon. in personal products such as denture , contact lenses, Even the most fervent advocates of PAM gels for tissue aug- diapers, and wound dressings. Injectable polyacrylamide gels are mentation acknowledge the existence of these adverse reactions, becoming increasingly popular for use in plastic and reconstruc- though they place blame on human error (i.e. contaminated tive surgery, especially for facial and breast augmentation. Pro- gel, improper technique, poor hygiene) as the underlying cause. ponents of this application claim that the gels are non-toxic and Other researchers blame the gel itself. Several researchers have stable. More recently, PAM hydrogels have been injected direct- noted that it may take several months to a few years for compli- ly into the urethral wall to treat stress incontinence in women. cations to arise; thus, the impact of injectable PAM gels must be Researchers state that polyacrylamide gel “seems to be a promis- studied over time. In fact, enough negative evidence now exists ing new bulking agent” in treating incontinence, despite the fact that usage of PAM hydrogels in tissue augmentation surgery that 16 of the 17 patients in the study had negative health events is forbidden in Russia and Bulgaria, where they had been used (such as urinary tract infection) associated with the treatment. since the early 1990s. Regardless of arguments regarding culpa- Alarmingly, there have been dozens of studies telling of hun- bility, researchers are increasingly recommending against its use dreds of patients with complications resulting from polyacryl- for facial tissue augmentation or tissues that have not been previ- amide injection, including pain, hematoma, nodule formation, ously operated upon. Still others call for prohibiting its use in gel migration resulting in tissue asymmetry or deformation, in- plastic surgery and searching for alternatives.

PAGE 36 • MASTERGARDENER • www.MasterGardenerOnline.com AUTUMN 2007 (Bacillus sphaericus and Acinetobacter the polymer, eliminating the possibility spp.) and white rot fungi (Dichomitus of acrylamide production (acrylamide Hydrogel squalens, Phanerochaete chrysosporium, contains nitrogen). Yet others have and Pleurotus ostreatus). The fungal argued that degrading gels do produce use for species dissolve the polymer, which is measurable levels of acrylamide, especially then susceptible to further degradation by when exposed to elevated temperatures or erosion many other soil microbes. high levels of solar radiation. In any case, It’s not surprising that polyacrylamide there is no question that PAM hydrogel control is rapidly broken down by decomposers; degradation produces uncharacterized, oluble polyacrylamide gels have been Sused for over a decade in reducing ero- one study found the average size of the variable polyacrylate units whose sion and enhancing water infiltration of fine- polymer to be less than 25 percent of environmental and human impacts are textured agricultural soils. Unlike “water the original in only 14 days of microbial unknown. crystals” that retain their shape as they ab- action. These gels contain a significant sorb water, soluble PAM dissolves in water, amount of nitrogen, which is often a Hydrogels and human health forming a thin slimy film that coats the soil limiting nutrient in both aerobic and There are two separate, but related, surface. In irrigation furrows and other bare anaerobic environments. human health issues relevant to PAM soils where irrigation can exacerbate erosion, this film protects the soil from washing away In most outdoor applications, hydrogels: risk of polyacrylamide and hydrates the surface so that irrigation therefore, the functional life of exposure and risk of acrylamide exposure. water can more easily permeate. polyacrylamides is short; this is borne The dangers from acrylamide exposure Many studies have been conducted on out by a number of studies that have were briefly mentioned earlier and tend a variety of soils in different environments, noted decreased efficacy of field-applied to be greatest for workers in occupations showing that agricultural PAMs are a viable polyacrylamide gels over time. If gel that routinely use polyacrylamide-based (though short-term) solution to soil loss and activity is destroyed in as little as products such as grouts and wastewater degradation. Often their effectiveness can 18 months, there should be serious flocculants. It is unlikely that the be enhanced by the addition of gypsum --a calcium source --especially in saline soils. reservations about its use in long-term infrequent user of garden hydrogels will Anionic soluble PAMs have generally been landscape applications. experience any significant exposure to found to be more effective than cationic acrylamide from this source. formulations in reducing soil erosion, which Gel contamination The other health issue is that is fortunate considering the environmental As the name suggests, polyacrylamides presented by exposure to the more or less toxicity of cationic gels. consist of many linked acrylamide units intact polyacrylamide gel. Though PAM Certain soils and environmental condi- (monomers). Acrylamide is a known gels are much less toxic than acrylamide, tions are antagonistic to soluble PAM gel effectiveness. In general, soluble PAM gels neurotoxin in humans and is suspected chronic exposure can cause minor do not work well on sandy soils, and can to be carcinogenic as well. During the problems such as skin irritation and actually reduce infiltration, possibly due manufacture of PAM gels, residual mucus membrane inflammation. More to pore blockage by the viscous gel. They acrylamide is present as a contaminant worrisome are recent reports of toxic may not work well on clay soils. Sodic soils and strictly regulated in the United States effects of PAM both at the cellular and decrease soluble PAM gel effectiveness since to levels of no more than 0.05 percent whole organism levels. An earlier article sodium prevents ion bridging and prevents or 500 parts per million for agricultural from 1992 reported on the accidental soil aggregation. Neither do they perform well on slopes, often increasing runoff, and use. However, an international study aspiration of polyacrylamide by a patient requiring either higher applications of gel or recommended that polyacrylamide gels who subsequently died from lung injuries. additional mulching materials to maintain used in cosmetics contain a residual Finally, there is the issue of exposure effectiveness. monomer level of only 0.1 to 0.5 to degrading PAM hydrogels, whose risks Usage of soluble PAM is acknowledged ppm. Therefore, the PAM hydrogels are entirely unknown. People most likely to be a short-term solution to erosion; for manufactured for agricultural and to be exposed to degradation products this reason its usage should not be extended garden use can contain much greater would be those involved in agricultural to garden and landscape use. Mulches are concentrations of toxic acrylamide than or nursery production where gels are demonstrably better in reducing erosion than bare soil and PAM hydrogel and pro- that found in personal products. commonly used and where environmental vide other, additional benefits. Moreover, While new PAM hydrogels contain degradation would be most likely to the soluble PAM hydrogels have no docu- a higher initial level of acrylamide occur. Compost piles containing potting mented benefit to plant growth. As with than older gels, there is bitter debate mixes with hydrogels would also be a many other agricultural production prac- over whether the degradation of source of exposure. tices, soluble PAM usage does not translate polyacrylamide gels provides a constant, Should you be concerned about well to home gardens and landscapes. significant source of environmental your exposure to PAM hydrogels? This is Soil scientists R.E. Sojka and R.D. Lentz have provided a concise and informative acrylamide. On one hand there are the where the big picture regarding hydrogel review of linear PAM applications in agricul- researchers who claim that microbes usage becomes important. Because these ture; it can be found at http://polymersinc. quickly metabolize the nitrogen from compounds are so ubiquitous, it’s likely com/polymers/pam2.htm.

AUTUMN 2007 www.MasterGardenerOnline.com • MASTERGARDENER • PAGE 37 Hydrogels are found in numerous products, including cosmetics, medical supplies, as well as soil additives. Jim Black that most of us are exposed to a number of PAM gel sources every day. Studies that estimate lifetime risks of developing cancer usually focus on only one source PAMs, however, have greater toxicities Recommendations of exposure, such as that from usage and should not be used. The charged Many of the products labeled “water of personal care products that contain nature of cationic PAM hydrogel is gel crystals” and “poly-clear” are cationic polyacrylamide. While these individual attracted to hemoglobin in fish gills, PAM gels. Not only are they more toxic estimates are almost always very low, where the gel binds and suffocates the to aquatic organisms and generally less there have not been analyses to determine fish. In addition to fish, a variety of effective than anionic gels in landscape additive risks associated with exposure to algal and invertebrate species are also situations, they can also contain higher multiple sources of polyacrylamide. The injured or killed when exposed to low levels of residual acrylamide. Even though lack of scientific data makes it difficult to levels of cationic PAMs. Since cationic these cationic gels are banned for many predict risks associated with exposure to PAMs may also contain higher levels of applications, they are still manufactured polyacrylamide gels. acrylamide monomer, many researchers and sold in the United States, China, and recommend against any environmental other countries. Cationic PAM hydrogels Environmental health use of cationic PAM hydrogels and in should not be used in gardens As a neurotoxin and carcinogen, fact use of these compounds is illegal in a and landscapes. acrylamide is dangerous not only to number of municipalities where aquatic It is difficult to predict short-term humans but to other organisms as well; contamination is likely. effectiveness of anionic PAM hydrogels the evidence does not need to be repeated on plant survival and establishment, since here. Of more concern for gardeners Alternative strategies the ability to absorb water is reduced by and landscapers is the impact of PAM The recognized hazards associated several environmental factors, especially hydrogels on other organisms in the with cationic PAM gels, as well as those salt, temperature extremes, ultraviolet environment. associated with residual acrylamide, have radiation, and microbial activity. The spurred many researchers to develop functional lifespan of cross-linked PAM Cultural practices that alternatives for agricultural and landscape hydrogels used outdoors can be as short usage. These alternatives include resins, as 18 months and at best only a few years; conserve soil moisture are paper-making by-products, and a number they cannot be regarded as long-term of polysaccharides such as gums, starches, solutions to landscape water needs. simple, inexpensive, safe, and gels. These alternatives are more As PAM gels degrade, they give environmentally sound, and in many rise to smaller, less functional polymers effective, and natural cases are cheaper to use and functionally whose risk to people and ecosystems is superior to polyacrylamide gels. unknown; they also produce some level alternatives. The best news for those of us of acrylamide, a known neurotoxin. Lack managing a home garden or landscape of documented information on the nature Microbes do not appear to be is that simple changes in management and toxicity of degraded PAM hydrogels negatively affected by PAM gels; indeed, practices are often superior to using makes it impossible to assess human or we already know that gels are colonized polyacrylamide hydrogels. In several cases, environmental health effects. People and degraded by a number of naturally alternative water management strategies need to be aware of their total exposure occurring bacteria and fungi. Toxicity had higher success rates than usage of to polyacrylamides from all sources, information on terrestrial organisms PAM. These strategies were as simple as including occupational use, garden (other than humans) exposed to PAM adding 2 liters of water when planting products, and cosmetics. gels is nearly non-existent and therefore Pinus patula seedings, or providing There are a number of products and can’t be addressed. Some inhabitants wind protection to reduce water stress management practices that can reduce of aquatic systems, however, have been in muskmelon. More commonly, unnecessary usage of and exposure to studied in relation to PAM gel toxicities mulches (especially organic) were rated polyacrylamide. In particular, cultural and the news is not good. superior to hydrogels in terms of erosion practices that conserve soil moisture are There are few indications that control, enhancing water infiltration simple, inexpensive, safe, effective, and anionic PAM gels, used appropriately, and conservation, plant growth and natural alternatives to PAM hydrogels. pose a significant health threat to establishment, and nutrient value. The hydrogel references are at: aquatic organisms. Cationic and neutral www.TheInformedGardener.com. n

PAGE 38 • MASTERGARDENER • www.MasterGardenerOnline.com AUTUMN 2007