Biodegradation testing of hydrophobic and volatile organic chemicals

Biodegradation data of chemicals are required during environmental risk assessments of chemicals under different regulatory frameworks. These data for regulatory needs are often generated using tiered laboratory biodegradation tests using standard OECD guidelines. The highest tiers are formed by environment simulated biodegradation tests such as in soil using (OECD 307), water-sediment (OECD 308), surface water (OECD 309) and activated sludge (OECD 314B).

These guidelines clearly states that they are not suitable for testing of volatile and highly hydrophobic chemicals. However, there are several chemicals across different regulations which have these properties, thus suitable tests are urgently requied for testing these chemicals. Abiotic losses of volatile and hydrophobic chemicals (i.e. evaporation and sorption) should be minimized in such tests, since such losses can compromize the validity of the test results (i.e. incomplete mass balance) and affect the numerical test results.

Although, the guidelines recommend the use of closed flask test setups for testing slightly volatile chemicals, no clear guidance on the test system geometry, headspace volume and monitoring/maintainance of aerobic conditions in such a closed setup is given. Furthermore, the degradation kinetics in such a test setup is largely influenced by volatilization and hence new data treatment measures are needed.

With regard to hydrophobic chemicals, they are difficult to introduce in aqueous tests and poses an analytical challange. These chemicals also have a high tendency to adsorb to test vessels, which leads to lowered bioavailability and hence also affects the degradation. The aims of this project were 1) to develop a test setup for testing of volatile chemicals across all these test guidelines and 2) to optimize alternative test chemical introduction techniques like dynamic passive dosing for testing of hydrophobic chemicals in aqueous tests (OECD 309).

Within this project, a basic closed flask test design for testing volatile chemicals in soil was initially developed and further this design was adapted for conducting other tests. With the developed test setups, complete mass balances for the range of volatile chemicals were obtained in full scale tests.

In particular, the use of co-solvent for the application of test chemicals had considerable influence on the oxygen levels within such setups. The oxygen monitoring data suggest that stable aerobic conditions was maintained in OECD 307, 309 and 314B studies. Whereas for OECD 308 oxygen diffusion from headspace to the water phase was limited.

This affected the test conditions within the test setup and further development of the test setup is therefore required. For testing of hydrophobic chemicals, a constant exposure concentration of test chemicals was maintained throughout the test using the optimized dynamic passive dosing approach. In comparison to the solvent spiked samples, higher degradation, volatilization and sorption was observed in passive dosed studies.

Although the sorbed and volatilized fractions linearly increased over time, total degradation i.e. the sum of metabolites and mineralization was initially linear followed by a plateau. As the degradation data from such tests can only be obtained based on product formation kinetics, separation of metabolites from parent chemical forms a critical step in these studies.

The dynamic passive dosing approach offered analytical advantages and demonstrated as a suitable approach also for testing semi-volatile chemicals and for obtaining mineralization data. Thus, the overall development and findings provides recommendations for obtaining reliable biodegradation data for hydrophobic and volatile chemicals which could be further used for their persistency assessment.