ASTM D6731 Standard Test Method for Determining the Aerobic, Aquatic Biodegradability of Lubricants or Lubricant Components in a Closed Respirometer
4. Summary of Test Method
4.1 Biodegradation of a lubricant or the component(s) of a lubricant is determined by measuring the oxygen consumed when the lubricant or component is exposed to microorganisms under controlled aerobic aquatic conditions. This value is then compared to the theoretical amount of oxygen (ThO2) which is required to oxidize all of the elements (that is, carbon, hydrogen, nitrogen, and so forth) in the test material. This test method mixes the test material (lubricant or component) with aerobic microorganisms in a closed respirometer containing a defined aquatic medium and measures the biodegradation of the test material by following the decrease in oxygen in the respirometer.
4.2 The test material is the sole source ofcarbon and energy in the medium. Areference material known to biodegrade, such as low erucic acid rapeseed oil (LEAR or canola oil) is run alongside the test material to confirm that the inoculum is viable and capable ofbiodegrading suitable materials under the test conditions. The test material or reference material concentration is normally 50 to 100 mg/L, providing a theoretical oxygen demand of at least 50 mg O2/L but no more than 200 mg O2/L. The ThO2 of the test and reference materials will be determined from measured elemental compositional analysis and will be calculated as in 13.1.
4.3 The inoculated medium is stirred in a closed flask and the consumption of oxygen is determined either by measuring the amount of oxygen required to maintain a constant gas volume in the respirometer flask, or by measuring the change in volume or pressure (or a combination of the two) in the apparatus.
4.4 Evolved CO2 (carbon dioxide) is absorbed in an alkaline trap solution (for example, 10 M NaOH or KOH) or other CO2 -absorbing system suspended within the test vessel, typically in the headspace of the test vessel.
4.5 Biodegradation is followed over a specified period by determining the consumption of oxygen. The amount of oxygen utilized in oxidation of the test and reference material is corrected for oxygen uptake by the inoculum in the blank controls and is expressed as a percentage of the theoretical oxygen demand (ThO2) calculated from the empirical formula of the material. Evaluation of the biodegradability of the test material is made on the basis of these data. Normally the test duration is 28 days; however, the test may be terminated if oxygen consumption has plateaued. The test may be extended as long as the systems' integrity is maintained and the inoculum in the blank systems is viable. The duration of the test will be dependent on the length oftime required for the rate of test material biodegradation to achieve a plateau. A graphical illustration ofthe test results for a biodegradable material is presented in Fig. 1.
5. Significance and Use
5.1 Results from this test method suggest the degree of aerobic, aquatic biodegradation of a lubricant or lubricant component. The rate and extent of oxygen consumption is measured upon exposure of the test material to an inoculum within the confines of a controlled laboratory setting. Test materials which achieve a high degree of biodegradation in this test may be assumed to easily biodegrade in many aerobic aquatic environments.
5.2 Because of the stringency of this test method, low results do not necessarily mean that the test material is not biodegradable under environmental conditions, but indicate that further testing is necessary to establish biodegradability.
5.3 If the pH value at the end of the test is outside the range from 6 to 8 and if the percentage degradation of the test material is less than 50 %, it is advisable to repeat the test with a lower concentration of the test material or a higher concentration of the buffer solution, or both.
5.4 A reference or control material known to biodegrade under the conditions ofthis test method is necessary in order to verify the activity of the inoculum. The test must be regarded as invalid and shall be repeated using a fresh inoculum if the reference material does not demonstrate biodegradation to the extent of >60 % of the ThO2 within 28 days.
5.5 Information on the toxicity of the test material to the inoculum may be useful in the interpretation of low biodegradation results. Toxicity ofthe test material to the inoculum may be evaluated by testing the test material in combination with the reference material in inhibition control systems. If an inhibition control is included, the test material is assumed to be inhibiting if the degradation percentage of the reference material is lower than 40 % (ISO 8192:1986). In this case, it is advisable to repeat the test with lower concentrations ofthe test material.
5.6 Total oxygen utilization in the blank at the end of the test exceeding 60 mg O2/L invalidates the test.
5.7 The water solubility or dispersibility of the lubricant or component may influence the results obtained and hence comparison of test results may be limited to lubricants or components with similar solubilities.
5.8 The behaviors of complex mixtures are not always consistent with the individual properties of the components. Test results for individual lubricant components may be suggestive ofwhether a mixture containing these components (that is, fully formulated lubricants) is biodegradable, but such information should be used judiciously.
