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Use the right term Many terms are associated with biodegradability: primary, ready/readily, ultimate/ultimately, potentially, inherently, simulation test, and abiotic transformation. These are all related to test methods and their meanings are clearly defined in OECD (Organisation for Economic Co-operation and Development) guidelines.
The primary biodegradation is the alteration in the chemical structure of a substance, brought about by biological action, resulting in the loss of a specific property of that substance. The laboratory tests which are carried out to evaluate primary biodegradation are mainly based on specific analysis of test substance die-away or concentration of intermediate substances formed. Though informative, a positive test result cannot, however, be considered as proof that the test substance will rapidly biodegrade in the environment.
In inherent, potential, and intrinsic biodegradability tests, degradation has a high chance of taking place. These test procedures allow prolonged exposure of the test substance to microorganisms and a low ratio of test substance to biomass, which offers a better chance of obtaining a positive result. Some of these tests may also be conducted using microorganisms that have previously been exposed to the test substance, which frequently results in adaptation, leading to a significant increase in the degradation rate. Because of the favourable conditions employed in these tests, rapid biodegradation in the environment of inherently biodegradable chemicals cannot generally be assumed.
Ultimate, ready, and easy aerobic biodegradation refers to the level of degradation achieved when the test substance is totally utilised by micro-organisms, resulting in the production of carbon dioxide, water, mineral salts and new microbial cellular constituents (biomass).
These tests are so stringent that positive results are unequivocal, and it may be assumed that the chemical will undergo rapid and ultimate biodegradation in the environment. In such cases, no further investigation of the biodegradability of the chemicals is normally required.
Simulation biodegradability tests simulate degradation in a specific environment with realistic conditions (indigenous biomass, temperature, soil, sediments or surfaces which allow the sorption of the chemical and a low concentration of test substance). Biodegradation is measured either by radiolabelling techniques or by specific chemical analyses.
Frequently, an important step during the degradation of chemicals in the environment is the abiotic degradation, which includes oxidation, photolysis and hydrolysis. Although abiotic transformation in itself is only primary degradation, the products formed by such abiotic processes may be biodegraded further by microorganisms.
Biodegradability principle and test methods A solution, or suspension, of the test substance in a mineral medium is inoculated and incubated during 28 days under aerobic conditions in the dark or in diffuse light.
The amount of dissolved organic carbon (DOC) in the test solution due to the inoculum should be kept as low as possible compared with the amount of organic carbon due to the test substance. Allowance is made for the endogenous activity of the inoculum by running parallel blanks with inoculum but without test substance, although the endogenous activity of cells in the presence of a chemical will not exactly match that in the endogenous control.
A reference compound is run in parallel to check the operation of the procedures.
In general, degradation is followed by the determination of parameters such as loss of DOC, CO2
production and oxygen uptake and
respirometry; with automatic respirometers, the measurement is continuous. DOC is sometimes measured in addition to another parameter, but this is usually done only at the beginning and end of the test. Specific chemical analysis can also be used to assess primary degradation of the test substance and to determine the concentration of any intermediate substances formed.
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LUBE MAGAZINE NO.176 AUGUST 2023
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