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Draft: State of the Science Report on the Bioaccumulation and Transformation of Decabromodiphenyl Ether

Appendix C - Selection of Log Kow for QSAR Modelling of decaBDE

The log Kow 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 Kow value of 1.845x106, or log Kow of 6.265. It is of interest that the log Kow 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 Kow, 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 Kow 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 Kow value. For these reasons, the log Kow 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 Kow of 9.97. Their approach, which applied to other PBDEs, showed a pattern of increased Kow with increasing bromination using a revese-phase HPLC method.

Overall, the range of reported and measured values of log Kow for decaBDE indicate a significant degree of uncertainty and measurement error. Log Kow 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 Kow. 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 Mark Bonnell, 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 Kow values generally above 8.0 (e.g., Wania and Dugani 2003). Wania and Dugani (2003) reported a log Kow value of 8.7 for decaBDE as the most reliable input for long-range transport modelling based on the establishment of a QSPR (quantitiative structure-property relationship) for higher brominated PBDEs and using the three solubility concept. This value was judged to represent a more realisitic log Kow value for modelling purposes and was used for bioaccumulation modelling in this report.

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