Reductive Dechlorination of Hexachlorobutadiene by a Pd/Fe Microparticle Suspension in Dissolved Lactic Acid Polymers: Degradation Mechanism and Kinetics

Abstract

Reductive dechlorination of hexachlorobutadiene (HCBD) was performed by a suspension of scattered spots of palladium nanoparticles deposited on iron microparticles (nPd/μZVI) in a mixture of dissolved lactic acid polymers and oligomers (referred to as PLA). The effects of nPd/μZVI loading, temperature, HCBD initial concentration, and PLA content were investigated as to define the best conditions for the dechlorination. HCBD dechlorination by nPd/μZVI occurred in a two-step process: first, HCBD adsorbed onto the nPd surface, which resulted in a rapid initial disappearance of pollutant in solution, and, second, it degraded chemically by atomic hydrogen H*, which resulted from the dissociative adsorption of H2 on nPd. HCBD remained adsorbed on the surface until its complete degradation in nonchlorinated product, in agreement with the formation of an ordered activated complex on the nPd/μZVI surface as suggested by the negative entropy of activation calculated from the Eyring equation. Hence, a minimum amount of nPd/μZVI was required to enable simultaneously HCBD adsorption and H2 production. In these cases, pseudo-first order rate equations were suitable to model HCBD disappearance kinetics. The increase in PLA content resulted in enhancing initial pH decrease such as to maintain acidic conditions and thus high reactivity over a longer period of time. It also resulted in enhancing the contact between HCBD and nPd/μZVI, which was characterized by a more important initial adsorption. As a consequence, deviations from pseudo-first order kinetics were observed and a more representative model with a two-phase decay was proposed.