Biodegradability
Biodegradability describes the ability of organic chemicals to be decomposed biologically, i.e. by living things (especially saprobionts ) or their enzymes . In the ideal case, this chemical metabolism runs completely through to mineralization , so that the organic compound is broken down into inorganic substances such as carbon dioxide , oxygen and ammonia , but the breakdown can also stop with transformation products that are stable to breakdown .
Different understanding
Biodegradability means very different things in practice, depending on whether reference is made to the statutory provisions or to further ideas. Industrially produced chemicals are considered to be biodegradable if they are removed from the environment through biological degradation and fed into the mineral cycle. This raises the question of how biodegradability is tested, how far the degradation is considered in the test and whether the necessary degradation conditions (e.g. temperature, other nutrients) exist at the respective location.
The association with private composting , which is common in common parlance , repeatedly leads to misunderstandings, see compostability .
Transformation products that arise as intermediate stages during the biological degradation of a substance can also be more problematic than the original substance . From alkylphenol polyglycol ethers , for example, nonylphenols are formed in the first degradation steps , which have hormonal effects that are harmful to fish . A distinction must also be made here between metabolites and transformation products. The former arise through metabolic transformation, primarily in humans, while transformation products include all possibilities for the transformation of a substance. Since abiotic processes (e.g. hydrolysis in water) cannot be ruled out when investigating biodegradability, the term transformation product is better for the resulting products.
The sometimes used, fuzzy expression “ elimination ” has nothing to do with biological degradation. An elimination of substances, for example in a sewage treatment plant, means that these substances can be detected in a lower concentration after the treatment than before the treatment . However, these substances can also be removed from the wastewater by transformation , adsorption , sedimentation or filtration . In the case of adsorption and sedimentation, however, the substances are bound in the sewage sludge and can therefore continue to be in the environment; in the case of filtration, it depends on the filtration unit, and in the case of transformation, the transformation products are introduced into the environment.
Test procedure
The guidelines for the testing of chemicals of the OECD , which are also used in connection with the approval of chemicals, are generally recognized . The biodegradability of water-insoluble substances, such as lubricating oils, can be determined with the specially developed test method CEC-L-33-A-93. There are still legally prescribed test procedures for certain surfactants that are less informative. For the classification as biodegradable plastic , the compostability is also examined.
Readily biodegradable (OECD 301)
The tests of the OECD test series 301 (A – F) demonstrate rapid and complete biological degradation under aerobic conditions. Different test methods are available for readily or poorly soluble as well as for volatile substances. If a substance does not meet the criterion for easy biological degradation (mostly more than 60% degradation), the inherent biological degradation is tested in the next hierarchical level (OECD 302).
- DOC Die Away Test (OECD 301 A) : The test substance is used in a high concentration compared to the other tests, i. H. 10–40 mgDOC / L (DOC = Dissolved Organic Carbon = dissolved organic carbon ) tested. The DOC concentration is measured at defined intervals over a period of 28 days. This test can be used for adsorbing substances.
- Carbon dioxide development test (OECD 301 B) : The carbon dioxide produced by the biological degradation of the test substance is analyzed regularly over 28 days and is an indicator of the biological degradation. This so-called Sturm test is used to examine chemicals that are poorly soluble in water .
- Modified MITI test (OECD 301 C) : With a special inoculum according to MITI specifications, the oxygen consumption is measured over a period of 28 days with simultaneous adsorption of carbon dioxide. This test is suitable for easily soluble substances.
- Closed bottle test (OECD 301 D) : The biodegradability of the test substance is determined by determining the consumption of dissolved oxygen at regular intervals over a period of 28 days. This test is used for volatile chemicals.
- Modified OECD screening test (OECD 301 E) : The biodegradability of the test substance is determined by measuring the DOC over a period of 28 days. This test is used when chemicals are sufficiently soluble in water.
- Manometric respiration test (OECD 301 F) : A defined gas space is left in the test bottles. In addition, a carbon dioxide adsorbent (e.g. sodium hydroxide) is placed in a special container in the test bottle. During the test, oxygen from the gas space is consumed and replaced by carbon dioxide, which is absorbed by the adsorbent . The resulting pressure difference is recorded by special pressure gauges and shows the biological degradation. This test is suitable if the molecular formula of the substance being examined is known and a theoretical oxygen consumption can be calculated with it.
Inherent degradability (OECD 302)
The tests of the OECD test series 302 (A – C) show that the chemical being examined is biodegradable to a limited extent, but it is fundamentally possible. Substances that pass such tests are generally or inherently biodegradable.
- The Zahn-Wellens EMPA test (OECD 302 B) examines the aerobic biodegradability of the test substance and gives the result of the decrease in chemical oxygen demand or the dissolved organic carbon. It is the most widely used test to study inherent degradability. It also provides information about the adsorption behavior of the examined substance.
Biodegradability of oil-soluble products (CEC-L-33-A-93)
In this method, a small amount of the oil to be tested is shaken for 21 days in an inoculated mineral substrate under constant conditions. Then it is extracted with Freon and analyzed by infrared spectrometry. By comparing the absorption of the infrared light of the untreated “zero-day approaches” with the absorption of the 21-day approaches, the biodegradable (biodegradable) portion of the oil can be determined. The method CEC-L-33-A-93 can only be used to determine the biodegradability of oil-soluble substances. It is not restricted to individual substances, but can also be used to determine the degradability of finished products.
Surfactant prescription test
Often there are manufacturer specifications on detergents and cleaning agents, according to which the surfactants used there are designated as biodegradable because they “fully meet all legal requirements regarding biodegradability”. The tests according to the EU directives 82/242 / EEC or 82/243 / EEC (or according to the tenside regulation for the German detergent and cleaning agent law ) only provide that anionic and non-ionic tensides are 80 percent detergent must be dismantled; Cationic surfactants are not taken into account in the statutory test procedures. The test procedures only examine primary degradation. Any further degradation of the test substance, as described in the OECD tests, is not considered here. For cationic and amphoteric surfactants , as for numerous other cleaning agents, there are no legally prescribed test regulations.
Compostability
Bioplastics are subjected to the compostability test of plastics . In Germany, this was described under the DIN standard DIN V 54900-1 ... 3, which was withdrawn without replacement in 2004 ; the American ASTM D-6400 requires plastics to be degradable by 60% within 180 days in order to label products as “compostable”. Recognition as a biodegradable material and compostable material only takes place if the substances are degraded to at least 90% within 12 weeks in an industrial composting process in accordance with European standard EN 13432.
Handling persistent properties
Substances are said to be resistant to degradation if they are not subject to biological degradation. If they are not decomposed by other chemical or physical degradation processes, they are referred to as persistent . Because of the persistence problem, in addition to chemical and photochemical degradation and adsorption, the biodegradability of substances released into the environment is of great importance. The biodegradability is therefore checked with the aforementioned OECD tests when chemicals are approved. According to Kümmerer, the most complete possible degradability (mineralization) after the application of the chemicals should be considered as part of the functionality for sustainable chemistry.
literature
- Franz Daschner (2006): Practical hospital hygiene and environmental protection Heidelberg, Springer; ISBN 3-540-61219-X .
- Thomas Kluge , (2002): Chemical Assessment - From Risk to Precautionary Concept. In: Günter Altner et al. (Ed.): Yearbook Ecology 2003 . Munich, CH Beck, 141-148; ISBN 3-406-47624-4 .
- Andreas Längin, Armin Schuster, Klaus Kümmerer: Chemicals in the Environment - the Need for a Clear Nomenclature: Parent Compounds, Metabolites, Transformation Products and Their Elimination. In: CLEAN Soil, Air, Water. 36, 2008, pp. 349-350, doi : 10.1002 / clen.200600001 .
- Klaus Kümmerer, Engelbert Schramm (2008): Drug development: The reduction of water pollution through targeted molecular design. In: Environmental Sciences & Pollutant Research 20 (4), 249–263.
- CEC Test Operation L-33, October 3, 1997.
- Bernd Beek (Ed.) (2001): Biodegradation and Persistence. Vol. 2K, The Handbook of Environmental Chemistry. Jumper; Berlin ISBN 3-540-62576-3 .