Transformations of polyhalogenated aliphatic compounds in homogeneous and heterogeneous model systems

• Main Author: Venkatapathy, Raghuraman
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2001
• Subjects: Environmental engineering
• ISBN: 0493297197; 9780493297194

Statistical Techniques were used to examine rate-limiting steps such as those due to diffusion, sorption or dissociative electron transfer in the transformation of polyhalogenated aliphatic compounds (PACs) in various homogeneous and heterogeneous model systems. The model systems are representative of engineered and natural environments where the PACs are reduced either in the aqueous phase or at the surface of metals, oxides or hydroxides. Linear Free Energy Relationships based on Marcus theory were generated for transformation of PACs in model aqueous systems containing bulk reductants and the electron transfer mediators iron porphyrin or mercaptojuglone. The results support earlier hypotheses based on PAC kinetic results, reaction intermediates, and products in the model systems that the initial, rate-limiting step in the reaction in both model systems is a dissociative one-electron transfer. A comparison of rate constants measured in heterogeneous systems from the literature with second-order rate constants measured in the homogeneous system containing iron porphyrin indicated that the rate-limiting step could be a dissociative one-electron transfer step. Results from statistical analyses indicate that the overall transformation of PACs in heterogeneous systems is limited by a combination of mass- and electron-transfer with the transformation of methanes being limited by electron transfer, that of ethenes by mass transfer, and that of ethanes by a combination of mass and electron transfer. Five models that account for experimental observations of kinetics and surface properties during trichloroethylene (TCE) transformation by iron were compared in terms of accuracy in fitting sixteen TCE concentration versus time profiles obtained from the literature. At low TCE loading, model results suggest that the overall transformation is reaction limited. At high TCE loading however, the overall transformation appears to be sorption limited due to competitive self-inhibition of TCE on the iron surface. The results suggest that in experiments initiated in the zero-order region, the order of the reaction may change to first-order and then back to zero-order as the reaction progresses due to a decrease in active site concentration as the catalytic surface is deactivated.