Ecological State of the Science Report on Decabromodiphenyl Ether (decaBDE)

The log K_ow for commercial DecaBDE of 6.265 determined at 25°C by CMABFRIP (1997a) is frequently cited in the literature. This value was determined using the generator column method with a composite material received from Great Lakes Chemical Corp., Albemarle Corp. and Bromine Compounds Ltd. and consisting of 97.4% decaBDE, 2.5% nonaBDE and 0.04% octaBDE. The analytical method used was GC-ECD and, using this method, the mean concentration measured in the aqueous material was 0.04 µg/L, while the mean concentration in octanol stock solution was 0.0738 g/L. This resulted in a K_ow value of 1.845x10⁶, or log K_ow of 6.265. It is of interest that the log K_ow determined for decaBDE by CMABFRIP was in fact lower than those which they determined for the commercial OctaBDE (6.29; CMABFRIP 1997b) and PentaBDE technical products (6.59; MacGregor and Nixon 1997) using the same procedure. This is not expected, due to the increased hydrophobicity and increased K_ow, which would be expected with increased bromination. This also does not reflect available information respecting commercial DecaBDE’s solubility in water. CMABFRIP (1997c) determined that the limit of water solubility was < 0.1 µg/L at 25°C, while for commercial OctaBDE (CMABFRIP 1997d) it was determined to be 0.5 µg/L, and 13.3 µg/L for commercial PentaBDE (Stenzel and Markly 1997). The available QSAR estimates for decaBDE indicate that it is predicted to be in the range of 2.841 x 10^-8 to 2.6078 x 10^-4 µg/L at 25°C. Thus, one can expect that difficulties measuring substances with such low solubility in water could have affected the determination of log K_ow by CMABFRIP (1997b). The European Communities (2002) also indicate that the presence of trace amounts of octanol in the water phase of this study may have had an apparent effect on the water concentration and the log K_ow value. For these reasons, the log K_ow for DecaBDE of 6.265 is considered too low and not appropriate for BAF and BMF modelling purposes.

Watanabe and Tatsukawa (1990) reported a significantly higher log K_ow of 9.97. Their approach, which applied to other PBDEs, showed a pattern of increased K_ow with increasing bromination using a reverse-phase HPLC method.

Overall, the range of reported and measured values of logK_ow for decaBDE indicate a significant degree of uncertainty and measurement error. Log K_ow measurements and estimates in this report range from 6.3 to 12.9. Other available estimates include 7.68, 10.37, 10.14, 9.44, 12.11 and 11.48 (from: www.vcclab.org/lab/alogps/start.html). The comprehensive Syracuse Research Corporation database used for the KOWWIN program contains 11 135 values for log K_ow. The maximum measured value in this training set is 11.3 and the upper 99.5 percentile is 7.8 (personal communication from JA Arnot to Environment Canada 15 February 2008; unreferenced). Therefore, most of the model estimates for decaBDE are greater than presently measured values for all other organic chemicals. Analytical limitations and high levels of measurement error are the principle reasons for lack of log K_ow values generally above 8.0 (e.g., Wania and Dugani 2003). Wania and Dugani (2003) reported a log K_ow value of 8.7 for decaBDE as the most reliable input for long-range transport modelling based on the establishment of a QSPR (quantitative structure-property relationship) for higher brominated PBDEs and using the three solubility concept. This value was judged to represent a more realistic logK_ow value for modelling purposes and was used for bioaccumulation modelling in this report. The logK_ow value of 8.7 was further used to adjust the predicted logK_ows of the expected metabolites of decaBDE using the Experimental Value Adjustment (EVA) method outlined in the KOWWIN program. The EVA method takes into account structural differences between the metabolite and parent compound (decaBDE) and adjusts the preferred value of 8.7 accordingly.