Mixed bromine/chlorine transformation products of tetrabromobisphenol A formed in the combustion of printed circuit boards: Emission characteristics and transformation pathways.

Recently, mixed bromine/chlorine transformation products of tetrabromobisphenol A (ClBrBPAs) were found to be possibly related to the thermal treatment processes of electronic wastes. To explore their emission characteristics and formation mechanism, printed circuit board scraps were combusted in a tube furnace, under the temperature from 25 °C to 600 °C. The emission factor of the debromination products of tetrabromobisphenol A (BrBPAs) was the highest, whereas that of ClBrBPAs was the lowest. Among three phases, most of the target compounds were partitioned into the oil and particle phases, and only negligible gaseous 2-BrBPA and bisphenol A were detected. The emission rates of most compounds were fastest at 300 °C, although 2-BrBPA, 2,6-BrBPA, and 2-Cl-6-BrBPA peaked at 350 °C. Among the chemicals in total emission, 2-BrBPA was the dominant congener in BrBPAs, whereas 2-Cl-2',6,6'-BrBPA, 2-Cl-2',6-BrBPA, and ΣClBrBPAs shared similar proportions in ClBrBPAs. Meanwhile, the composition profiles at 300 °C showed that 2,2',6-BrBPA and 2-Cl-2',6,6'-BrBPA occupied the largest proportions in BrBPAs and ClBrBPAs, respectively. To reveal the possible transformation pathways, the Gibbs free energy was calculated based on a radical substitution reaction. After "•Br" removal from tetrabromobisphenol A or other BrBPAs, the intermediate was more easily combined with "•H" than with "•Cl." In addition, the ClBrBPA formation via "-•H + •Cl" by BrBPAs is nonspontaneous, thus limiting the further generation of ClBrBPAs. This study not only provides ideas for the study of other mixed halogenated products, but also provides constructive suggestions for environmental source analysis by combining previous research on the occurrence of ClBrBPAs in various environmental matrices.