Because of the favourable conditions employed in these tests, a rapid biodegradation in the environment of inherently biodegradable chemicals cannot generally be assumed. However, if an inherent test is negative this could indicate the potential for environmental persistance of the organic compound.
2.2. Abiotic transformation Chemicals in aquatic environments, soil and air may be transformed by abiotic processes such as transformation can be an important step in the pathway for degradation of chemicals in the environment. Although abiotic transformation in itself is only primary degradation, the products formed by such abiotic processes may be biodegraded further by microorganisms.
2.3. Ultimate biodegradability test methods First, the aerobic biodegradability should be examined in a screening test for ready Biodegradability.
In case of a negative result in a test for ready biodegradability, the biodegradation of the chemical may be examined in a simulation test to obtain data describing the biodegradation rate in the environment. Alternatively or supplementary a screening test for inherent biodegradability may be conducted for generation of data describing the potential biodegradability under optimised aerobic conditions, such as those which may potentially occur in biological sewage treatment plants.
Since inherent biodegradability can be considered to be a specific property of a chemical, it is not necessary to define limits on test duration or biodegradation rates.
For ultimate Biodegradation, a rates above 20% may be regarded as evidence of potential, primary biodegradability, whereas biodegradation rates the pass levels of either 60% (ThOD or ThCO2) or
70% DOC practically represent complete ultimate degradation of the test substance as the remaining fraction of 30-40% of the test substance is assumed to be assimilated by the biomass or present as products of biosynthesis.
Biodegradability test methods description A solution, or suspension, of the test substance in a mineral medium is inoculated and incubated under aerobic conditions in the dark or in diffuse light.
The amount of 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 non- specific parameters such as DOC, CO2 production and oxygen uptake and measurements are taken at sufficiently frequent intervals to allow the identification of the beginning and end of biodegradation. 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. It is obligatory in the MITI method (301 C).
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The most requested tests for lubricants are carried out according to OECD 301B and OECD 301F; details oft he test methods are given hereafter :
The modified sturm test according to OECD 301B is based on the same principle that methods ISO 9439, NFT 90-306 and ASTM D 5864. The degradation is followed by the carbon dioxide
production.The test substance is introduced by an appropriate means at a concentration between 10 and 20 mg of organic carbon per liter of test medium. The air used must be CO2-free. Following biodegradation reaction, the organic carbon is finally transformed in CO2; this CO2 is entertained by the air and trapped in a Barium hydroxide solution which is in excess. Through a back titration, the remaining barium hydroxide is titrated by hydrochloric acid with phenolphtalein as the colorimetric indicator.
Ba(OH)2 excess + CO2 g BaCO3 + Ba(OH)2 Ba(OH)2 + 2HCl g BaCl2 + 2H2O
The CO2
production is evaluated every 2-3 days at the beginning of the test, then every 4-5 days at the end of the test or when biodegradation reachs a plateau.
Normally, the test lasts for 28 days. Tests however may be ended before 28 days, i.e. as soon as the biodegradation curve has reached a plateau for at least three determinations. Tests may also be prolonged beyond 28 days when the curve shows that biodegradation has started but that the plateau has not been reached by day 28, but in such cases the chemical would not be classed as readily biodegradable.
A summary of the test methods principle is included in the table hereafter:
LUBE MAGAZINE NO.126 APRIL 2015
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