Polybrominated biphenyls (PBBs) are toxic and persistent brominated flame retardant. Yet their debromination by zero valent metal (ZVM) or iron based bimetals has received far less attention. Here we reported the kinetics and debromination pathways of PBBs in nanoscale zerovalent iron (n-ZVI), microscale zerovalent zinc (m-ZVZ), n-ZVI/Ag and n-ZVI/Pd systems. The results show that the singly occupied molecular orbital (SOMO) of PBB anion can be well correlated with the debromination pathways of PBBs by n-ZVI, which verify its ability to predict their debromination pathways through electron transfer (e-transfer) mechanism. We also found that the reaction rates of PBBs in m-ZVZ, n-ZVI/Ag and n-ZVI/Pd systems are much faster than that in n-ZVI system, and the debromination pathways of PBBs in n-ZVI, m-ZVZ and n-ZVI/Ag are exactly the same, suggesting that n-ZVI/Ag debrominate PBBs through an e-transfer mechanism. However, Pd was found to be able to utilize H-2 to debrominate PBBs and the debromination pathways of PBBs in Pd-H-2 system was the same to those in n-ZVI/Pd system, but was partially different from those in n-ZVI, m-ZVZ and n-ZVI/Ag systems, suggesting that n-ZVI/Pd debrominate PBBs through a H-atom transfer mechanism.