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Biodegradation tests on fully formulated lubricants are relevant due to the high number of single components, which make testing each individual component costly and impractical. Moreover, the lubricant formulation is a mixture of base oils with some additive packages. It’s very unusual that the additives manufacturer provides the pure active substance, which will have a different chemical structure that the base oil in which it is diluted; so, the same situation with the additive package and with the formulated lubricants arises.
We must also remember the scope of application of the biodegradability test methods, which forbids carrying out the test at a concentration that is toxic for the micro-organisms. If you consider the lubricant as a single component, taking into account the concentration of the active additive substance in the lubricant and the test concentration, the additive molecules may not be toxic while testing the fully formulated lubricant.
Finally, in relation to eco-toxicity, if the lubricant is in contact with the environment, this concerns the entire lubricant and not single components. The test methods applicable to lubricants and greases, based on CO2
production and O2
the Total Organic Carbon (TOC) and the Theoretical Oxygen Demand (ThOD) respectively are known.
The scope of application for both methods is limited to organic compounds which are not toxic for micro-organisms at test concentration. Moreover, the inoculum must not come from industrial sewage treatment, nor be pre-adapted (pre-exposed) to the test substance.
Test methods based on CO2 production are not
applicable to volatile compounds and those which contain more than 5% of inorganic carbon (like carbonates) within the total carbon content.
Test validity A test is considered valid if: • •
• Test uncertainty
Biodegradability tests use biological materials composed of inoculums from different sources and origins and which are not normalised. This is certainly one of the key reasons for results variation.
Poorly water-soluble organic compounds also require various modes of preparation, which further increases variation.
The accumulation of uncertainties of analytical determinations from the pre-requested data to the 28-day end period further impacts test variation and leads to a reproducibility as high as 30%.
It is worth noting that variation is smaller for test substances which present high or low biodegradation results.
To limit variation, the best practice for comparing the biodegradability of many test substances is to carry out the tests in the same series, using the same inoculum and the same mode of preparation; in this case, test variation can usually be reduced to less to 10%.
consumption, require that
Interpretation of test results Threshold level
The duration of 28 days for ready biodegradability tests was defined in order to allow sufficient time for the microorganisms to adapt to the chemical. The pass level for theoretical carbon dioxide (ThCO2
)
and for theoretical oxygen demand (ThOD) is 60%; this level is considered evidence of ready biodegradability.
According to OECD guidelines, reaching the pass level means it may be assumed that the chemical will undergo rapid and ultimate biodegradation in the environment.
The reference reaches the pass level at day 14
In case of toxicity test, the biodegradation is above 25% at day 14
• The cumulated CO2
The difference between replicates at the plateau (or the end of the 10-d window) is lower than 20% content from the inoculum
and air is lower than max 40 mg 24 LUBE MAGAZINE NO.176 AUGUST 2023
Biodegradation above 20% may be regarded as evidence of inherent, primary biodegradability. When results of ready biodegradability tests indicate that the pass level criterion is almost fulfilled (i.e., ThOD or ThCO2
slightly below 60%), such results can be used to indicate inherent biodegradability.
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