Reductive debromination of polybrominated diphenyl ethers by zerovalent iron

Polybrominated diphenyl ethers (PBDEs) are a new class of global, persistent, and toxic contaminants, which need proper remediation technologies. PBDE degradation in the environment is not well understood. In this study, degradation of PBDEs with zerovalent iron was investigated with six BDEs, substituted with one to 10 bromines. Within 40 days 92% of BDE congener 209 was transformed into lower bromo congeners. During the initial reaction period of BDE 209 (< 5 days), hexa- to heptabromo BDEs were the most abundant products, but tetra- to pentabromo congeners were dominant after 2 weeks. The amount of mono- to tribromo BDEs was steadily increased during the experiments. BDEs 28, 47, 66, and 100 also showed a stepwise accumulation of lower bromo congeners. No oxidation products were detected in all experiments. The results showed that a stepwise debromination from n-bromo-to (n-1)-bromodiphenyl ethers was the dominant reaction in all congeners. The reaction rate constants of lower bromo BDEs decreased as the number of bromines decreased. The initial reductive debromination rate constants were positively correlated with the heats of formation of BDEs. The preferential accumulation of specific congeners was observed in the experiment with BDEs 28, 47, 66, and 100, where the most abundant products were BDEs 15, 28, 37, and 47, respectively. Reactions proceeded to form more stable and less brominated products that have lower heats of formation. Almost all the possible isomers from a specific parent BDE were found in all the experiments, which was probably due to the small difference of heat of formation between the products (2-5 kcal/mol). Reactions of all congeners proceeded fast at the initial phase (< 5 days) followed by a slow reaction. The rate of reductive debromination of BDE 209 was slower with environmentally relevant sulfide minerals (iron sulfide and sodium sulfide). However, the product congener pattern, produced by sulfide mineral catalysis, was nearly similar with that of zerovalent iron treatment. This may be a possible source of lower brominated BDEs in the environment. Debromination of PBDEs by zerovalent iron has high potential values for remediation of PBDEs in the environment.