New method for investigating polymers in the environment
When sewage sludge is used as an agricultural fertilizer, the polyacrylamide copolymers (PAMs) that are used as flocculants are released into the environment. According to the Fertilizer Ordinance, these cannot remain in the soil. Fraunhofer researchers have developed a method to test under realistic conditions whether PAMs degrade in the environment and how quickly this happens. The study has found that the degree of degradation complies with the requirements of the Fertilizer Ordinance. The new method can also be used in other areas to investigate the persistence and degradation of polymers and microplastics in the environment.
According to the Fertilizer Ordinance, which regulates in Germany »the marketing of fertilizers, soil additives, culture substrates and plant auxiliaries«, synthetic polymers, including polyacrylamide copolymers, must not remain in nature once they have been introduced into the environment. PAMs must therefore be able to degrade naturally in the environment without the use of any further additives. The ordinance stipulates that PAMs degrade by at least 20 percent within two years. However, the ordinance does not specify a procedure for verifying this value.
In a three-year study, researchers from the Fraunhofer Institute for Applied Polymer Research IAP and their colleagues at the Fraunhofer Institute for Molecular Biology and Applied Ecology IME investigated the behavior of PAMs under natural conditions. They developed a completely new method that takes into account the special conditions that arise when sewage sludge is deposited into the soil: »Various studies have investigated the degradation of PAMs under UV radiation. However, this method does not work in the case of fertilizers, because the material is plowed into the soil where UV light cannot get to it,« explains Dr. Erik Wischerhoff from the project team at the Fraunhofer IAP. »Therefore, we had to find out whether the PAMs degrade under these conditions. We developed a new testing method for this purpose.«
How quickly do polyacrylamide copolymers degrade in the soil?
For this study, the Fraunhofer team used radical polymerization to produce radioactively labelled PAMs and conducted field trials. For three years, soil samples were taken every six months and subjected to a special extraction procedure. This ensured that all of the polymers, which were partially bound and finely distributed in the soil, were measured.
Gel permeation chromatography was then used to determine the molar masses of the PAMs found in the extract. Radioactive labelling makes these analyses possible. The results provided information on how strongly the polymers had degraded. The special thing about this is that the Fraunhofer researchers used controlled radical polymerization to produce the necessary reference polymers, so-called standards. Their structure is similar to that of the extracted polymers, thus enabling reliable conclusions to be drawn.
»Our study found that the polyacrylamide copolymers degraded by more than 20 percent within two years and thus meet the requirements of the Fertilizer Ordinance. The study did not investigate precisely how the PAMs degrade. However, it is clear that the long carbon chain, characteristic to these polymers, breaks down into smaller pieces. This was investigated in more detail in a separate project,« says Wischerhoff.
The scientists have published the results of their work in the international journal »Environmental Sciences Europe«. The Fertilizer Advisory Council of the Federal Ministry of Food and Agriculture has yet to release their assessment of whether the results can be used as a general basis for evaluating the use of sewage sludge treated with PAMs as a fertilizer.
Author: Julia Schmidt