ReviewPolymer biodegradation: Mechanisms and estimation techniques – A review
Introduction
The respect of the environment is a capital point in a sustainable development context. We should act in this way to preserve fossil resources and reduce the pollution of the Earth. The fabrication of industrial products must consume less energy and the raw materials must be in priority renewable resources, in particular from agricultural origins.
Currently, two approaches are explored to minimise the impact of the usage of polymers on the environment:
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The design of polymeric materials for long duration (e.g. aeronautic devices, construction materials, coatings and containers), these materials must combine unalterability and be fashioned preferentially from renewable resources (e.g. plant oil in thermoset, wood fiber in composites materials) (Wuambua et al., 2003, Mougin, 2006, Sudin and Swamy, 2006, Ashori, 2008). This kind of materials of industrial interest and low environmental impact is not within the aim of this review due to a minor biodegradability.
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Technological innovations designed for the production of polymers for short duration (e.g. disposable packages, agricultural mulches, horticultural pots, etc.) (Bastioli, 1998, Chandra and Rustgi, 1998, Lörcks, 1998, Lunt, 1998, Averous and Le Digabel, 2006) must have the intention of fast biodegradability. Most biodegradable polymers belong to thermoplastics (e.g. poly(lactic acid), poly(hydroxyalkanoate), poly(vinyl alcohol)) or plants polymers (e.g. cellulose and starch). Thermoplastics from polyolefins are not biodegradable, even if some of them have prooxidant additives making them photo and/or thermodegradable, the assimilation of oligomers or monomers by microorganims is not yet totally proved.
This dichotomy between durable and biodegradable polymers is not obvious. In recent years, innovating experiments are realised to combine both approaches, the results are the production of polymeric materials with controlled life spans. The designed materials must be resistant during their use and must have biodegradable properties at the end of their useful life. A possibility to obtain interesting results is to co-extrude natural and artificial polymers, in order to combine the properties of each macromolecule to obtain the desired properties (Muller et al., 2001, Shibata et al., 2006). Today, a fast-growing industrial competition is established for the production of a great variety of controlled life span materials. It is important to develop new comparative tests to estimate their biodegradability. Actually, it seems to have confusion in the interpretation of biodegradation, biofragmentation and biodeterioration. Hereafter, we are giving attention to the meaning of polymer biodegradation.
Earlier, biodegradation was defined as a decomposition of substances by the action of microorganisms. This action leads to the recycle of carbon, the mineralisation (CO2, H2O and salts) of organic compounds and the generation of new biomass (Dommergues and Mangenot, 1972). At present, the complexity of biodegradation is better understood and cannot be easily summarised (Grima, 2002, Belal, 2003). The biodegradation of polymeric materials includes several steps and the process can stop at each stage (Pelmont, 1995) (Fig. 1)
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The combined action of microbial communities, other decomposer organisms or/and abiotic factors fragment the biodegradable materials into tiny fractions. This step is called biodeterioration (Eggins and Oxley, 2001, Walsh, 2001).
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Microorganisms secrete catalytic agents (i.e. enzymes and free radicals) able to cleave polymeric molecules reducing progressively their molecular weight. This process generates oligomers, dimers and monomers. This step is called depolymerisation.
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Some molecules are recognised by receptors of microbial cells and can go across the plasmic membrane. The other molecules stay in the extracellular surroundings and can be the object of different modifications.
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In the cytoplasm, transported molecules integrate the microbial metabolism to produce energy, new biomass, storage vesicles and numerous primary and secondary metabolites. This step is called assimilation.
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Concomitantly, some simple and complex metabolites may be excreted and reach the extracellular surroundings (e.g. organic acids, aldehydes, terpens, antibiotics, etc.). Simple molecules as CO2, N2, CH4, H2O and different salts from intracellular metabolites that are completely oxidised are released in the environment. This stage is called mineralisation.
The term “biodegradation” indicates the predominance of biological activity in this phenomenon. However, in nature, biotic and abiotic factors act synergistically to decompose organic matter. Several studies about biodegradation of some polymers show that the abiotic degradation precedes microbial assimilation (Kister et al., 2000, Proikakis et al., 2006). Consequently, the abiotic degradation must not be neglected.
Herein, we describe the different degrees of the biodegradation process: biodeterioration, biofragmentation and assimilation including the abiotic involvement. Each mechanism is illustrated by an example. Furthermore, we suggest the technical estimation adapted to each level of biodegradation.
Section snippets
Abiotic involvement
Polymeric materials that are exposed to outdoor conditions (i.e. weather, ageing and burying) can undergo transformations (mechanical, light, thermal, and chemical) more or less important. This exposure changes the ability of the polymeric materials to be biodegraded. In most cases, abiotic parameters contribute to weaken the polymeric structure, and in this way favour undesirable alterations (Helbling et al., 2006, Ipekoglu et al., 2007). Sometimes, these abiotic parameters are useful either
Biodeterioration
Deterioration is a superficial degradation that modifies mechanical, physical and chemical properties of a given material. Abiotic effects provoking deterioration are described above. This section focuses on the biological aspects of deterioration.
The biodeterioration is mainly the result of the activity of microorganisms growing on the surface or/and inside a given material (Hueck, 2001, Walsh, 2001). Microorganisms act by mechanical, chemical and/or enzymatic means (Gu, 2003).
Microbial
Biofragmentation
Fragmentation is a lytic phenomenon necessary for the subsequent event called assimilation (cf. § Assimilation). A polymer is a molecule with a high molecular weight, unable to cross the cell wall and/or cytoplasmic membrane. It is indispensable to cleave several bonds to obtain a mixture of oligomers and/or monomers. The energy to accomplish scissions may be of different origins: thermal, light, mechanical, chemical and/or biological. The abiotic involvement was described previously. This
Assimilation
The assimilation is the unique event in which there is a real integration of atoms from fragments of polymeric materials inside microbial cells. This integration brings to microorganisms the necessary sources of energy, electrons and elements (i.e. carbon, nitrogen, oxygen, phosphorus, sulphur and so forth) for the formation of the cell structure. Assimilation allows microorganisms to growth and to reproduce while consuming nutrient substrate (e.g. polymeric materials) from the environment.
Conclusion
The biodegradation is a natural complex phenomenon. Nature-like experiments are difficult to realise in laboratory due to the great number of parameters occurring during the biogeochemical recycling. Actually, all these parameters cannot be entirely reproduced and controlled in vitro. Particularly, the diversity and efficiency of microbial communities (e.g. the complex structure of microbial biofilm) and catalytic abilities to use and to transform a variety of nutrients cannot be anticipated.
Acknowledgements
The authors are grateful to the ADEME (Agence de l’Environnement et de la Maîtrise de l’Energie) for its financial support and thank particularly M. BEWA for his advices.
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