Aquatic Microbial Toxicity of Organophosphorous Flame Retardants Assessed with Ready Biodegradability Testing Methods

Abstract

The objective of this M.Sc. research project, conducted in the framework of ENFIRO, a project within the Cooperation Program of the EU 7th Research Framework Programme, was to assess the aquatic toxicity of organophosphorous flame retardants (FRs) tetraphenyl resorcinol bis(diphenylphosphate) (RDP), bisphenol- A bis(diphenylphosphate) (BDP). This was done by testing the inhibitory effect of FRs on the degradation of glucose. In the first experiment Guideline 301 of the Organisation for Economic Co-operation and Development (OECD) for ready biodegradability testing was applied and CO2 production was measured continuously using the automatic respirometer Respicond VI. To overcome the challenge of obtaining a constant dissolved concentration of the strongly hydrophobic substances, RDP and BDP were loaded onto, and passively dosed to the inoculums from PDMS silicone devices. The loading of the PDMS passive dosing devices was unsuccessful; only 1.87% and 2.41% of the intended loaded mass of RDP and RDP was found to be present on the PDMS tubing respectively. Although the automated respirometer Respicond VI has proven to be convenient in use, it has also shown to be prone to contamination when used for liquid samples. In the second experiment, applying the head space test (OECD guideline 310), the CO 2 evolution was measured with gas chromatography. The concentrations of saturated RDP and BDP solutions were shown to increase with dilution factor. Also, sedimentation occurred after ~10 days. Rather than a true solution, the solutions therefore likely were colloidal micro-emulsions. In neither experiment a decrease in degradation rate or ultimate percentage degradation with increasing FR concentration was observed. The percentage degradation of glucose in the first experiment was 90.05±4.03% for RDP and 86.03±3.83% for BDP, and in the second experiment was 39.79±6.16% RDP and 34.91±4.56% on average for BDP. Thus, no toxicity effect was observed. However, from these data no conclusions can be drawn about the FRs toxicity; no measurable concentrations of the target FR were present in the Respicond study. In the head space test, the test medium likely was a micro-emulsion rather than a true solution, which must certainly have had implications for the FR bioavailability to the test organisms. Further, the pass level of 60% degradation was never reached for any of the samples nor for the controls, and the test is therefore invalid. In the absence of a reliable means to obtain a constant aquatic concentration of RDP and BDP, the toxicity and persistence tests as designed are not likely to be successful in the future. Different research strategies may be considered; passive dosing may be optimized for use with these phosphorous based compounds, or the hypothesis that RDP and BDP form aggregates may be confirmed, after which these substances may be investigated further as colloid particles.