Superabsorbent polymer performance in agriculture and environment impacts
Superabsorbent polymer performance considerations
Superabsorbent polymer performance is going to be dependent on field conditions, application depth, and rate of application. Since SAPs will be incorporated with soil, it is important to consider their absorbency under load. With increasing BD and application depth, SAPs absorption time increased and overall absorbency decreased (Lejcus et al., 2018) and in addition, confinement by the soil matrix restricted SAP function (Yu et al., 2011). Salt type and concentration within the soil system can further diminish SAP absorbency (Al Jabri et al., 2015). These changes in capacity should be taken into consideration when determining SAP application method, rate of application, and depth of placement.
Some common concerns about the use and application of SAPs include safety, toxicity, and environmental fate. Superabsorbent polymers are “irreversible,” meaning that they cannot be returned to starting materials or converted to toxic metabolites (Zohuriaan-Mehr et al, 2008). The majority of the material safety data sheets for SAPs classify these compounds as being pH neutral, inert, and overall “Safe and Non-toxic,” and are currently not regulated as hazardous materials. A few examples include (a) Zap Zorb Crosslinked Sodium polyacrylate (9003-04-7, Zappa Tec LLC) is only an irritant if exposed over the recommended amount (8 h exposure limit of 0.05 mg m−3), and (b) composted polyacrylate is labeled as nontoxic to aquatic or terrestrial organisms at predicted exposure levels from current product specific application rates.
When SAPs are degraded in the soil, the products are CO2, water, ammonia, and other constituents such as Na depending on polymer composition (De Barros et al., 2017). While the products of degradation are not toxic, there is limited understanding in the breakdown mechanisms of these compounds and the cumulative effects, if any, that may occur within the soil ecosystem. The general half-life of SAPs in soil is 5−7 yr (Ekebafe et al., 2011), and their longevity will be influenced by soil pH (Sadd, Lopes, & Dos Santos, 2009). The stable C backbone, low solubility, and high mass due to cross-linking make SAPs resistant to breakdown by bacteria (Stahl, Cameron, Haselbach, & Aust, 2000). Biodegradation can occur as mineralization and solubilization, which can be aided by white rot fungi (Stahl et al., 2000). If the application of SAPs in soil for the above-mentioned uses increases, its degradation and longevity in soil may need further exploration.
Given that SAPs do not degrade readily and persist for several years, it is essential to understand their mobility. If SAPs become mobile their effectiveness may diminish and they could potentially enter the groundwater system. Even though these materials are non-toxic, there is potential concern if these materials enter the ground water. When 30 pore volumes of water were passed through sand test columns, 99.7% of polyacrylate absorbent and 92% of linear poly acrylic acid were retained within the column (Sack et al., 1998). Mobility of SAPs may be dictated by soil texture and organic matter. Soils with higher organic matter and clay contents will likely limit SAP mobility. Long-term studies of SAP mobility have not been conducted.
Superabsorbent polymer performance may change over time as a result of field conditions. These changes have been studied within the laboratory and on relatively short time scales. When thinking about applying these materials at the field scale, the longevity of performance over the course of multiple years needs to be evaluated. The degradation of the SAPs under various field conditions should be studied. Depth, rate, and method of application should all be evaluated at the field scale to determine benefit/cost ratios for the different types of SAPs. Overall, SAPs need to be examined in the field over longer periods, as well as how soil mechanical properties are changed.
Keywords: superabsorbent, super absorbent polymer, SAP powder
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