Biodegradation Process of Polymers
4.1 The Need of Ecotoxicity Analysis for Biodegradable Materials
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Biodegradable materials on the other hand are designed to fulfi l the specifi c needs of an application they are intended for and to become mineralised by microorganisms present in the environment or at a treatment facility for organic waste. Therefore biodegradable polymers will have a strong interaction with the ecosystem. They will become feed stock for the autochthone microorganisms and degradation residues and metabolites may be produced or enriched at that location.
Figure 4.1 gives an overview of the possible pathways of biodegradable materials. The most signifi cant deviation is between biodegradation due to organic recovery of biowaste and biodegradation in the environment. The fi rst is a combined thermo- and mesophilic process in presence of a dense population of microorganisms that are supported by watering and aeration. The second, the same in terrestrial or aquatic environments, is a biodegradation at meso- or psychrophilic conditions achieved by a less dense population of microorganisms and without active support. Therefore both types of biodegradation should be described separately. When designing an artifi cial biodegradable polymer where the degradation process will occur should be considered – this should preferably be at a waste treatment facility or in the environment.
Incineration and landfi ll as additional treatment techniques for residual waste are mentioned for completeness. The content of pollutants and harmful substances in residual waste is almost always determined by waste fractions other than biodegradable materials.
Therefore the established rules for incineration and landfi ll will cover biodegradable polymers well; additional considerations of ecotoxicological impacts are not needed.
4.1.1 Standards and Regulations for Testing of Biodegradable Polymers During the last few years some national and international standards, such as DIN 54900 [1], ASTM D6002:1996 [2] and EN 13432 [3], have been published. The intended goal of them all is to provide producers and users as well as authorities with test schemes and quality criteria (pass levels) for biodegradable materials. The three standards are different in detail but have the same basic four-step test scheme:
1. Estimation of the possibility of biodegradability based on the chemical composition (polymer structure) and absence of intentionally added components which are known to be or are under suspect of becoming toxic or harmful to the environment (for example heavy metals)
2. Determination of the degradability caused by microbial activity and quantifi cation by either oxygen demand, carbon dioxide release or methane production considering the time needed for full mineralisation
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105 3. Determination of the disintegration under real or simulated composting or anaerobic
digestion conditions and quantifi cation by gravimetric determination of a sieve residue 4. Investigation of the quality of the compost resulting from the material disintegration
test by analysis of chemical and physical parameters and by determination of ecotoxicological effects to at least higher plants
Figure 4.1 Pathways of biodegradable polymers and their main interactions with different ecosystems related to the intended use
Biodegradable polymer
Tools for agriculture &
fi shery
Packaging Other
products
Litter
Biowaste (source separated)
Residual waste
Soil &
sediment
Compost (organic recovery)
Landfi ll &
incineration
Water &
sediment Soil
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The standards discussed do consider a utilisation of biodegradable materials by regular waste processing and do not deal with procedures and criteria for degradation in soil or aquatic ecosystems. Nevertheless, they are trend setting by defi ning the fi nal compost as product. Every product has a market, a value and a circle of consumers and therefore needs quality criteria for regulation of both the price and the potential uses. All three standards contain the requirement that any introduction of man-made polymers into the established organic waste recovery must not negatively infl uence the quality of the fi nal compost.
It is easy to extend that philosophy to natural terrestrial and aquatic ecosystems although the specifi c standards are currently not fi nished: Man-made polymers must not have any negative infl uence on the environment to which they are applied. It should be acknowledged that any local ecosystem is in a natural balance that is worth protecting. It is expected that the standards in progress will follow that philosophy.
During the past decades, conventional agriculture has caused an almost global contamination of groundwater with residues of fertilisers (especially nitrate) and pesticides.
The protection of water resources, not necessarily drinking water alone, is one more aspect to be considered when biodegradable polymers are released to the environment in vast amounts. Finally national laws and regulations for drinking water have to be followed.
Natural ecosystems which are not of commercial interest are the least protected in all the previously mentioned examples, because only a very few standards and regulations could be applied to them. Nevertheless, they should be remembered when environmental effects of human intervention on the nature are discussed.
4.1.2 Detection of the Infl uences on an Ecosystem Caused by the Biodegradation of Polymers
As already mentioned, compost is a product under governmental quality regulation. It seems to be an easy task when declaring an unwanted effect caused by the degradation of polymers as any change of the quality relevant parameters. Since the composition of compost is dependent upon the composition of the original biowaste, all of the analysed chemical or biological parameters of the fi nal product will vary over a broad range. For determining the infl uence of biodegradable materials two test batches will be needed, one with the test polymer added and another without any additions. The physical and chemical parameters as well as ecotoxicological effects can then be compared between those two compost products.
Nevertheless, the interpretation of differences between the two batches could be very diffi cult.
How should a change in the pH-value (for example from 7.9 to 8.2), or in the plant available
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107 fractions of nutrients in relation to their total content be interpreted? A more readily applied result could be obtained from bioassays, since any decrease of the germination rate of the seed or any reduction of plant growth (production of green plant biomass) could easily be declared as a loss of compost quality, independent from the chemical composition.
Table 4.1 gives an overview of the environmentally relevant quality criteria defi ned in the three standards. It is important to mention that the avoidance of quality losses and negative impacts to small ecosystems starts with the material design. Known toxic substances and others, which are suspected to become harmful to the environment during or after the degradation process, must not be used. The bioassays included are a safety net to detect all those stable biodegradation residues and metabolites which may be formed by microorganisms and which were not present in the original polymer.
No standards are available to deal with the changes in the quality of soil and aquatic ecosystems caused by biodegradation of polymers. Again, the intentions of the standards for organic recovery should be extrapolated to those environments too. Since the chemical compositions of different soils as well as of aquatic ecosystems and their sediments could be very different from each other, the defi nition of acceptable changes will cause many more problems than for compost. For those ecosystems the application of biotests will be the most important impact control for biodegradable polymers.
Table 4.1 Quality criteria of the steps 2 and 4 defi ned in the analysis schemes for biodegradable materials
Standard Material design Criteria after composting DIN 54900 (1997) [1] Maximum 50% minerals
Maximum 30% of heavy metals allowed in compost
RAL-criteria [4] for chemical composition
Ecotoxicity tests with summer barley
ASTM D6002 (1996) [2]
No special requirements National US standards for chemical composition
Ecotoxicity tests with three plant species, earthworms and rotifers EN 13432 (2001) [3] No toxic or harmful
substances
Maximum 50% minerals Maximum 50% of heavy metals allowed in compost
European national standards for chemical composition
Ecotoxicity tests with two plant species
RAL: Reichs-Ausschuss für Lieferbedingungen
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4.1.3 Potential Infl uences of Polymers After Composting
The positive effects of the application of compost in agriculture are described in numerous publications, for example Allison [5], Voelker [6], Gottschall [7] and Hartl [8]. The most common effect is the fertilisation by mineral nutrition elements. Further Sekhon and Meelu [9] mention the supporting effect of organic matter on the micro-fl ora and on physical properties of less fertile soil.
Potential hazards caused by the introduction of toxic components (especially heavy metals) with contaminated compost are revealed very often. These typical impurities are covered by the analytical quality control of most of the national regulations in Europe and will lead to a classifi cation as second or third quality and to a limited use of the compost.
These national standards are dealing with well-known contaminants that may derive from typical biowaste and are focussing on heavy metals and a handful of halogenated or aromatic hydrocarbons. The inclusion of bioassays with higher plants in some standards is more to determine the maturity of the compost than with the appearance of ecotoxic effects caused by anything other than the chemicals being determined.
The collection of new, artifi cial, biodegradable materials together with the traditional biowaste and their composting (or anaerobic digestion) includes new risks of the introduction or generation of not known and therefore not analysed substances. Hope-Simpson [10] demonstrated for the fi rst time that residues from composting of coated paper could be toxic to plants and make it impossible to use such a compost in agriculture.
The reason was an enrichment of the nutrient element boron to a toxic level.
Insam [11] gives a very comprehensive overview about accepted test methods for investigations of biodegradable packaging for their suitability for various established composting processes. The methods are focussed on the determination of the degradability but do consider biotests as routine quality control. Also Pagga [12] does consider biotests as necessary quality control for compost batches containing degraded artifi cial packaging polymers.
The term compost quality should not be limited to physical and chemical parameters.
While such analysis could describe the contents of nutrients and the presence of a small number of selected pollutants, the appearance of unidentifi ed metabolites and residues could be detected more reliably by the application of biotests.
The increased cost for the additional analysis, before the introduction of materials on the market, will be rewarded by the confi dence of compost operators and compost users.
Mandatory biotests are needed during the phase of material development and are not necessary as an additional routine quality control of each batch of compost.
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109 The inclusion of mandatory ecotoxicity tests in the most relevant standards is already realised (Table 4.1). The extent of the investigations may differ between the standards, but they share the same intention: the detection of negative infl uences, which are not covered by the routine chemical analysis. Because of lack of practical experience, especially about how to deal with complex matrices like compost in conventional bioassays, not many mandatory methods are currently listed in the German and in the European standard.
Nevertheless, an option to include more or maybe specially adapted methods is kept in the EN 13432 [3].
4.1.4 Potential Infl uences of Polymers During and After Biodegradation in Soil and Sediment
It is a justifi ed claim that artifi cial materials should not inhibit the growth and crop yield of agricultural plants. That is valid for a short-term view as well as for a longer-term evaluation. Negative effects should neither appear during the same vegetation period that a biodegradable material is applied nor at the following years. The long-term observation is necessary, because repeated applications of biodegradable polymers may lead to an accumulation of potentially ecotoxic substances which are below the no effect concentration level (NOEC) after one single application.
In the standards dealing with organic recovery of biodegradable materials the necessity of ecotoxicological investigations is clearly stated. Not only because of the possible formation of unknown metabolites but also because of the behaviour of additives in polymers (for example conventional softeners) which are already know to be problematic.
This does not change if instead of a thermophilic composting process the slower, meso- or psychrophilic biodegradation in soil, in aquatic environments or in their sediments is observed. As is well known, the thermally initiated hydrolysis step of ester or ether bonds could be crucial for following the uptake of the built oligo- and monomers into the cells. Also for degradation at ambient temperature the fi rst hydrolysis step must either be catalysed due to microbial activity or initiated by other physical and chemical forces (for example by sunlight or by oxidation in presence of air). The probability of the appearance of undegraded residues and of their further accumulation in soil and sediments is increased compared to a thermophilic composting process.
On the other hand in many other publications the positive effects of organic substances in soil are described. Harvest residues (green plants), organic fertiliser (stable manure) and other organic, biodegradable substances can contribute positively to the physical structure and will therefore indirectly increase the soil’s fertility. Higher water holding capacity and elevated ion exchange capacity are the most often claimed causes for such
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improvements of the soil quality. Further a prospering soil micro-fl ora (applied with compost or grown due to biodegradation of plant residues) may induce disease resistance of plants. All those agricultural publications do consider only natural polymers, such as starch, cellulose, ligno-cellulose (wood), proteins and fats. The positive infl uences on plant growth, crop yield and quality are explained by the increased content of organic matter (humic substances) in the soil by several authors (Danneberg [13], Dick and McCoy [14], Gottschall [7], Hartl [8] and Knafl [15]). At least Sekhon and Meelu [9] do correlate the content of organic matter directly with the soil fertility.
Also very often negative effects are described, caused by the presence of biodegradable substances in soil and appearing during the time of plant growth. The most prominent is the formation of toxic fermentation by-products released in the early stages during the biodegradation of organic substances. This phenomenon is described by Lynch and co-workers [16] and Toussan and co-workers [17] and is well known in relation to the incorporation of crop residues in soil. The prime reason for reduced plant growth is the generally increased microbial activity, which may further lead to a drop in the pH-value and to an abnormal high oxygen demand, as described by Subba Rao [18] and Alloway and co-workers [19]. All these effects are of a temporary nature and will end soon after the biodegradation is completed. Other negative impacts are explained by the mobilisation of heavy metals, which are already present in the soil (or in the compost). While metals, which are bound to or are included in the mineral matrix, behave inertly in the ecosystem, the mobile and therefore bioavailable fractions can cause serious harm to plants and animals and can accumulate in the food chain (excerpt from Förstner and co-workers [20], Scrudato and co-workers [21] and Suffet and MacCarthy [22]).
The effects and infl uences caused by the deposition of communal residual and specifi c industrial wastes are well described in the literature. More literature is available detailing the ecosystem responses to known organic pollutants (mostly pesticides and polycyclic aromatic hydrocarbons) or their residues and metabolites during biodegradation. Almost no literature is available concerning the interaction between the biodegradation of organic substances, the appearance and the related mechanisms of non-reversible ecotoxicological effects. Related empirical and research results dealing with material of other than biogenic origin are missing in the literature. If such artifi cial polymers are completely biodegradable it could be assumed that they might not cause effects which are basically different from those of plant residues.
Although agricultural production plants are at the centre of interest when discussing ecotoxicological effects, other soil and water organisms should be included as well for an extension to a broad ecological assessment. Coleman and Crossley [23] claim that many commercially uninteresting organisms are an essential part of soil and determine its long time fertility.
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