Theoretical Study of the Mutual Degradative Interaction Between Elemental Mercury and Polyhalogenated Biphenyls

Prospect of pollutant-for-pollutant treatment strategy was theoretically investigated in this work by studying the mutual degradative interaction between elemental mercury (Hg⁰) and selected polychlorinated and polybrominated biphenyls (PCB128 and PBB128) using a range-corrected density functional theory (DFT) method, ωB97XD with SDD basis set. The reactions were modelled in vacuum, CHX₃, CH₂X₂ and HX (X = Cl & Br) and in the presence of HOX as co-reductant to determine the most suitable medium for maximal degradability. Kinetic and thermodynamic predictions revealed that the interactions are possible in vacuum but with minimal kinetic and thermodynamic feasibility. However, feasibility increased generally with an increase in medium polarity in the order of vacuum < CHX₃ < CH₂X₂ < HX. Also, kinetic and thermodynamic parameters suggested that PBB128 might be a better oxidant for the removal of Hg⁰ in the gas phase. Apart from consistency with existing reports (especially in vacuum and CHX₃), our results revealed for the first time the likelihood of a solvent-assisted transition from a rate-limiting first step to a rate-limiting second step during a zero valent metal’s reaction with polyhalogenated biphenyls. We believe these findings could cause significant paradigm shift in the management of wastes containing mercury, PCBs, PBBs and related contaminants.