Catalytic Role of Palladium and Relative Reactivity of Substituted Chlorines during Adsorption and Treatment of PCBs on Reactive Activated Carbon

The adsorption-mediated dechlorination of polychlorinated biphenyls (PCBs) is a unique feature of reactive activated carbon (RAC). Here, we address the RAC system, containing a tunable amount of Fe as a primary electron donor coupled with Pd as an electrochemical catalyst to potentially respond to the characteristic of contaminated sites, effectively traps and treats various PCB congeners. A dramatic increase in RAC reactivity with Pd doping at as low as 0.01% suggests its critical role for accelerating hydrodechlorination of PCBs. Characteristic adsorption and dechlorination behavior and ensuing decomposition pathways of 13 selected PCB congeners are discussed with their surface interactions with RAC. Important findings include W inherent dechlorination susceptibility of chlorines in pare > meta > ortho position, regardless of independent or competitive conditions as well as substrate effects, (ii) favorable reduction of more toxic coplanar PCB congeners, (iii) preferential electrophilic attack to chlorines in a less substituted phenyl ring and an isolated chlorine, regardless of the steric or inductive effect as a dominant limiting factor for the dechlorination of ortho or meta PCBs, respectively, (iv) prominent dechlorination inhibition for higher ortho congeners but negligible inhibition for higher meta congeners, and (v) eventual complete dechlorination of higher PCB congeners to biphenyl.