Kinetics of Tetrachloroethylene-Reductive Dechlorination Catalyzed by Vitamin B12

Abstract

Abstract-Reductive dechlorination kinetics of tetrachloroethylene (PCE) to ethylene catalyzed by vitamin B 12 using Ti [III] citrate as the bulk reductant was examined in a vapor/water batch system. A kinetic model incorporating substrate-B 12 electron-transfer complex formation and subsequent product release was developed. The model also accounted for the primary reductive dechlorination pathways (hydrogenolysis and reductive ␤ elimination) and vapor/water-phase partitioning. Reaction rate constants were sequentially determined by fitting the model to experimental kinetic data while moving upward through consecutive reaction pathways. The release of product from the complex was found to be second order with respect to substrate concentration for both PCE and acetylene; all other substrates appeared to release by first order. Reductive ␤ elimination was found to be a significant reaction pathway for trichloroethylene (TCE), and chloroacetylene was observed as a reactive intermediate. Acetylene production appears to be primarily due to the reduction of chloroacetylene derived from TCE. The reduction of cis-dichloroethylene (cis-DCE), the primary DCE isomer formed, was extremely slow, leading to a significant buildup of cis-DCE. The kinetics of acetylene and vinyl chloride reduction appeared to be limited by the formation of relatively stable substrate-B 12 complexes. The relatively simple model examined appears to adequately represent the main features of the experimental data.

Fenton process for degradation of selected chlorinated aliphatic hydrocarbons exemplified by trichloroethylene, 1,1-dichloroethylene and chloroform

“…Burris et al [11] investigated the reductive dechlorination of PCE, TCE, cis-DCE, and vinyl chloride (VC) catalyzed by vitamin B 12 using Ti [III] citrate as a bulk reductant in a vapor/water batch system. They reported that the reductive dechlorination proceeded very slowly, and many partially dechlorinated compounds were formed as the terminal byproducts.…”

Metalloporphyrin solubility: A trigger for catalyzing reductive dechlorination of tetrachloroethylene

“…One approach to degrade such pollutants is to use porphyrins as electrontransfer mediators to catalyze their reductive dechlorination. During the past two decades, a number of studies have used porphyrinogens or related tetrapyrrole macrocycles for the reductive dechlorination of a variety of chlorinated organic contaminants, ranging from carbon tetrachloride (see, e.g., [1][2][3][4][5][6]) to chlorinated ethanes and ethylenes (see, e.g., [7][8][9][10][11]) to polychlorinated biphenyls [12].…”

“…Catalyzed reductive dechlorination reactions have been reported for several soluble porphyrins and porphyrin deriva- tives, including metallotetrakis (N-methyl-4-4-pyridiniumyl) porphyrin (TMPyP) [1,13,14], protoporphyrin, uroporphyrin, coproporphyrin, and hematoporphyrin [28]; coenzyme F 430 [16]; hematin [17]; and most often, the corrin vitamin B 12 [3,4,[7][8][9][10][11]16,18]. Additionally, a variety of core metals have been used as the central ion of reactive metalloporphyrins, including cobalt [1], iron [13], nickel [14], and magnesium as well as molybdenum [28].…”

Role of metalloporphyrin core metals in the mediated reductive dechlorination of tetrachloroethylene

“…A particularly promising approach to abiotically dechlorinate such pollutants is to use porphyrins or other tetrapyrrole macrocycles as electron transfer mediators/shuttles to catalyze reductive dechlorination under anaerobic conditions. In the past two decades, a number of studies have reported utilizing tet-rapyrrole macrocycles for the reductive dechlorination of a variety of chlorinated organic contaminants, ranging from carbon tetrachloride [1][2][3][4][5][6][7] and chlorinated ethanes and ethylenes [8][9][10][11] to chlorobenzenes and PCBs [12,13] and other reducible oxidized pollutants [14][15][16].…”