Reductive Debromination of Polybrominated Diphenyl Ethers: Dependence on Br Number of the Br-Rich Phenyl Ring

Reductive debromination has been widely studied for the degradation of polybrominated diphenyl ethers (PBDEs), although the reaction mechanisms are not so clear. In the present study, the photocatalytic degradation and debromination of ten PBDEs were carried out with CuO/TiO₂ nanocomposites as the photocatalyst under an anaerobic condition. The pseudo-first-order rate constants were obtained for the photocatalytic debromination of PBDEs, and their relative rate constants ( k_R) were evaluated against k_R= 1 for BDE209. Unlike the generally accepted summary that k_R is dependent on the total Br number ( N) of PBDEs, k_R is found to depend on the Br number on a phenyl ring with more Br atoms than the other one. In other words, a phenyl ring substituted by more Br is more reactive for the reductive debromination. The calculated LUMO energies ( E_LUMO) of all PBDEs are well correlated to the more reactive phenyl ring with more Br, compared with the N of two phenyl rings. The result was explained by LUMO localization on the Br-rich phenyl ring, suggesting that the reductive debromination occurs on the phenyl ring.