Ca2+ mediated mechanism of octa-brominated dioxin/furan formation via BDE-209 thermolysis: Introducing the Mayer bond order difference.

Polybrominated dibenzo-p-dioxins/dibenzofurans (PBDD/Fs) that form during industrial thermal processes, such as the cement kiln co-processing of BDE-209, are highly toxic contaminants. Nevertheless, the formation mechanisms of octa-brominated dioxins/furans (OBDD/Fs), most PBDD/F congeners, and one precursor of the more toxic lower PBDD/Fs from BDE-209 have received little attention. In cement kiln co-processes, the Ca2+-mediated regulation of OBDD/F formation is still debated. In this study, simulation experiments revealed that the average brominating degree of PBDD/Fs was 7.8, indicating that OBDD/Fs are dominant congeners (93.6 % median). Density functional theory (DFT) calculations found a new transition state (TS1) with a lower energy barrier than that found in a previous study. Three major OBDD/F formation reactions suggested that the presence of Ca2+ was thermodynamically beneficial to the formation of OBDD/Fs. This promotion effect can be attributed to the transfer of electron density leading to a change in the Mayer bond order (MBO) among elements when Ca2+ was bound. Intriguingly, in the transition state structures of the Ca2+-bound and Ca2+-free systems, the MBO difference among the old and new bonds can reveal the difficulty of Ca2+-mediated OBDD/F formation reactions from BDE-209.