Mechanism of the degradation of individual PCB congeners using mechanically alloyed Mg/Pd in methanol.

Polychlorinated biphenyls (PCBs) are a continuing concern in the environment, although legislation restricting the production and use of PCBs was introduced more than 30 years ago. The combination of zero-valent metals and hydrogenation catalysts has been proven effective in the remediation of PCBs, although the exact mechanism of degradation is not known as of yet. The use of mechanically alloyed zero-valent magnesium and palladium (on graphite) has shown great success in the dechlorination of PCBs. Knowing the mechanism for this dechlorination would be helpful in optimizing the bimetallic Mg/Pd for use in the field. A variety of experiments have been performed on a single PCB congener (PCB-151, 2,2',3,5,5',6-polychlorobiphenyl) in an attempt to determine the mechanism by which the degradation occurs. The studies are carried out in methanol to mimic the solvent system which will be used in field applications. Results of these studies have suggested three possible mechanisms, all of which include the removal of the chlorine atom by a hydrogen as the rate-limiting step, varying only in the exact nature of the hydrogen species (radical, hydride, or "hydride-like" radical). BRIEF: A series of studies has suggested three possible mechanistic pathways for the degradation of PCBs in methanol by Mg/Pd.

Dechlorination comparison of mono-substituted PCBs with Mg/Pd in different solvent systems.

It is widely recognized that polychlorinated biphenyls (PCBs) are a dangerous environmental pollutant. Even though the use and production of PCBs have been restricted, heavy industrial use has made them a wide-spread environmental issue today. Dehalogenation using zero-valent metals has been a promising avenue of research for the remediation of chlorinated compounds and other contaminants that are present in the environment. However, zero-valent metals by themselves have shown little capability of dechlorinating polychlorinated biphenyls (PCBs). Mechanically alloying the metal with a catalyst, such as palladium, creates a bimetallic system capable of dechlorinating PCBs very rapidly to biphenyl. This study primarily aims to evaluate the effects of solvent specificity on the kinetics of mono-substituted PCBs, in an attempt to determine the mechanism of degradation. Rate constants and final byproducts were determined for the contaminant systems in both water and methanol, and significant differences in the relative rates of reaction were observed between the two solvents.