Biodegradation Process of Polymers
4.5 Research Results for Ecotoxicity Testing of Biodegradable Polymers
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be possible. The recommendation of Korte [73] to summarise single test results into an ecotoxicological profi le could be set for the analysis of biodegradation effects as well.
From a set of standardised biotest results a relative index could be calculated representing something like an ‘average’ ecotoxic hazard. This has been common practice for the risk evaluation of chemicals and pesticides for a long time [78]. Nevertheless, some diffi culties arise since the importance (the relative weight) of each single test result is not the same for different matrices and application scenarios. An example may demonstrate this: Hund [66] has found the earthworm test to be one of the most sensitive for testing of chemicals in soil. But Fritz found that the sensitivity of earthworms to be almost zero in presence of digestible substances (residues) after biodegradation tests [64].
Much more research will be necessary to characterise potential negative effects to the environment deriving from polymers and their degradation products with the needed accuracy and raggedness. What is listed here should be seen as a fi rst step.
131 behaviour of degradation end products. He states that polymers containing elements other than carbon, hydrogen and oxygen should be analysed more extensively for the appearance of unwanted effects after the biodegradation process. Halogens or heavy metals, for example, which are often introduced with pigments, may form by-products during the degradation process, which are not acceptable for environmental reasons.
But even pure hydrocarbons can cause ecological problems if they have an incompatible chemical structure. The relationship between the biodegradability of aliphatic and aromatic hydrocarbons is addressed by Müller [81]. Biodegradability is not simply correlated with the degree of polymerisation; additional factors like availability of the polymeric bonds for extracellular bacterial enzymes and the content of aromatic monomers do have major infl uence. Further the distribution of aromatic monomers in the polymeric chain does determine the degree of biodegradability and therefore indirectly the appearance of probably toxic residues.
4.5.2 Ecotoxicity of the Polymers
Dang and co-workers [82, 83], discuss the measurement of toxic effects of biodegradable polymers. Using a cell culture test system the authors demonstrated the functional suitability of the method with four samples. The results have been determined mainly by the presence of leachable substances, effects of non soluble polymers have not been observed.
The work of Stacher [65] has demonstrated that a direct measurement of the plant toxicity of biodegradable materials is impossible. Initiated degradation processes in the test trays had caused a dramatically reduced plant growth. Those effects appeared even if natural polymers, such as starch or cellulose were used and even if the soil was sterilised before the test. Tests with not readily biodegradable polymers should be possible since the microorganism community is not activated that much. In such a case ecotoxic effects will most probably be related to an incompatibility of leachable components from the material. That outcome should be considered for the conduction of bioassays with plants and with other test species as well.
The earthworm (Eisenia foetida) is defi nitely not suitable for determining the ecotoxicity of materials that are digestible by the animals. The feeding effect will result in an increased growth, which may more than compensate potential inhibition effects [65].
Aquatic bioassays made with elutriates from polymeric materials are possible although limited to water-soluble components. The elutrition procedure should be designed properly to simulate the conditions at the natural environment of the application. Examples could be the continuous elutrition in aquatic environments or a periodic exposure at times of
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rainfall in terrestrial environments. However, neither a standardised nor an otherwise validated method is currently available for such investigations.
4.5.3 Ecotoxic Effects Appearing After Degradation in Compost or After Anaerobic Digestion
Both these processes for the organic recovery of waste are fi nally very similar although the degradation pathways are different in principle. At least both are technical processes in an artifi cial environment producing compost as a main product. The compost pile and the digestion sludge do not need an analysis for ecotoxic effects caused by the introduction of biodegradable polymers. Guidelines for the detection of disturbances in the processes are already covered in some of the test scheme standards. Marketable mature compost, which is used in high quantities in agriculture, is the sample to be analysed for ecotoxic effects.
The analytical detection of residues and metabolites had been possible from a laboratory degradation test using a mineral bed matrix. Tosin and co-workers [84] described such a test system and the detection of the metabolite, 4,4´-diaminodiphenylmethane (a known toxic substance) as result of the degradation of polyurethane caprolactone co-polymer.
Especially the problems arising with high concentrations of organic substances in the matrix (humic substances) could be avoided. But it may be diffi cult to detect metabolites from a certain polymer when it is not known what to search for. The success of such methods and analysis procedures as a primary source of information may therefore not be assured. Nevertheless, such investigations could be helpful to discover the causes of ecotoxic effects already observed at other experiments.
The use of bioassays will give the most relevant information about the appearance of negative effects in compost. With all the limitations about test species and other known infl uences some data have been generated by Fritz and co-workers [68] analysing a set of commercially available polymers. Single results from several bioassays as well as their summary into an ecotoxicity profi le can be the most proper data base to get an impression about the infl uence of degradation residues in the compost on a complex ecosystem.
Figure 4.8 gives a summary of the results.
4.5.4 Ecotoxic Effects Appearing During Degradation in Soil
From a theoretical point of view, inherently biodegradable polymers should not behave other than dead biomass (leave litter, wood, whole plants) which is a signifi cant part of the natural carbon cycle. General effects of degradable substances on physical and chemical soil properties as well as on the soil ecology are described by Coleman [23]. They are almost always of positive nature distinct as a long time increase of productivity and soil fertility.
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133 The detection of ecotoxic effects deriving from degradation residues and metabolites is demonstrated with some examples in the Figures 4.9 and 4.10. Both, the initial suppression effect of readily degradable substances as well as the existence of additional ecotoxic effects could be demonstrated in those experiments. From Figure 4.9 it can be seen that the addition of biopolymers improved the original soil quality after the biodegradation had been completed. In summary, neither the application of one single species test nor the measurement at only one time of the degradation experiment will be enough to differentiate between the both mentioned effects.