Determination of synthetic lubricant concentrations in soil during laboratory‐based biodegradation studies

The use of synthetic, biodegradable lubricants for two-stroke engines has become widespread due to increased concern over the build-up of hydrocarbons, derived from mineral oil-based lubricants, in the aquatic environment. A method for assessing the biodegradation potential of new lubricants was developed by the Co-ordinating European Council (CEC) for the Development of Performance Tests for Lubricants and Engine Fuels in 1982. However, this method deals only with the aquatic environment. The use of such lubricants in agricultural machinery led to a modification of the CEC method to assess the relative biodegradability of lubricants in soil. After incubation of oil-treated soil in a simple and inexpensive incubation assembly, samples were extracted with solvent and the level of lubricant determined by infrared spectroscopy. The method was found to be both accurate and reproducible, giving excellent recoveries. It compared favourably with a gravimetric method and was so sensitive that it gave appreciable blank values from untreated soils owing to their content of long chain aliphatic natural products. The incubation and extraction methods were used successfully to assess the biodegradability of a range of synthetic lubricants in soil in comparison to a mineral oil-based lubricant and a natural vegetable oil. The synthetic oils were found to degrade more rapidly and extensively than the mineral oil-based lubricant but not as rapidly or extensively as the vegetable oil. Amendment with nitrate considerably enhanced the loss of all the oil tested, indicating the importance of nitrogen availability when assessing the inherent biodegradability of hydrocarbons in soil. As the data generated is from laboratory-based studies the results do not necessarily translate directly to the field situation. However, the method is suitable for screening new materials and for comparative purposes.