Coupling of natural organic matter–metal binding and laccase-catalyzed oxidation of tetrabromobisphenol A

Laccases are a group of copper-containing oxidase enzymes found in aquatic and terrestrial environment. They can catalyze one-electron oxidation of phenolic compounds to radical intermediates using molecular oxygen as the electron accepter. The radical intermediates can subsequently couple to each other to form dimers. In this study, we investigated the kinetics of tetrabromobisphenol A (TBBPA) transformation in laccase-catalyzed oxidation process. It was revealed that the removal of TBBPA was first order to the concentrations of both substrate and laccase. Natural organic matter (NOM) inhibited the reaction by reversing the oxidation of TBBPA. Such inhibition effect was more significant in the presence of Ca2+, Mg2+, Cd2+, Mn2+, and Co2+, but not Na+ or K+. This was because of the formation of NOM–metal complexes. Binding to metal ions neutralizes the negative charge of NOM, making it easier to access laccase molecules and thus have a greater chance to react with the radical intermediates. A numerical model that couples the laccase-catalyzed oxidation and NOM–metal-binding processes was constructed. This model successfully described the transformation of TBBPA in the presence of NOM and divalent metal ions in laccase-catalyzed oxidation process. Product identification indicated radical coupling and elimination was the main pathway of TBBPA transformation. Overall, this work provides important sights into the laccase-catalyzed oxidation process.