Correlation Analysis of Rate Constants for Dechlorination by Zero-Valent Iron

Abstract

Dehalogenation is among the most important processes involved in contaminant fate, but despite all the work that has been done on the kinetics of dehalogenation, there are few linear free energy relationships (LFERs) that can be used to explain or predict rates of dehalogenation by environmental reductants. Previously, we summarized kinetic data for dehalogenation of chlorinated alkanes and alkenes by zero-valent iron (Fe0) and showed that correlation analysis of these data with published two-electron reduction potentials did not give a simple relationship. In this study, we report successful LFERs based on estimated lowest unoccupied molecular orbital (LUMO) energies calculated from semiempirical (AM1 and PM3) and ab initio methods (6-31G*) and one-electron reduction potentials. Solvation effects can be modeled with COSMO and incorporated into semiempirical estimates of E LUMO, but this did not improve the correlation with k. The best LFER (log k = −5.7−1.5 E LUMO) explains 83% of the variability in surface area-normalized rate constants (k) with ab initio LUMO energies. The LFER is improved by correcting for statistical bias introduced by back transformation from log-linear regression models. New kinetic data for six compounds are compared with rate constants predicted using the unbiased LFER.

Dechlorination of Trichloroacetic Acid Using a Noble Metal-Free Graphene–Cu Foam Electrode via Direct Cathodic Reduction and Atomic H*

“…It was recognized that the strong electron withdrawing ability of the chlorine atom caused the carbon center electron deficient, which was more amenable to electrophilic attack. 29,30 Thus, the highly chlorinated CAAs would be more prone to be reduced during the dechlorination process, which is consistent with previous studies. Inset is the equivalent electrical circuit used to fit the EIS data (R e is the electrolyte resistance, R ct is the charge-transfer resistance, CPE is the constant phase-angle element, and R SEI and C SEI are the pseudocapacitance and resistance of solid/electrolyte interface, respectively).…”

Degradation of tetrachloromethane and tetrachloroethene by Ni/Fe bimetallic nanoparticles

“…However, many challenges still exist for implementation of the zero-valent metal technology, such as slow reaction rate, accumulation of persistent byproducts, and progressive deactivation of the ZVI by the precipitated corrosion products [6][7][8]. To overcome these, various nanoscale bimetallic particles have been synthesized and used to improve the performance of the ZVI technology [5] [7][8][9][10][11][12]. Nanoscale metal particles, with diameter in the range 1~100 nm, were characterized by high surface area to volume ratio, high level of stepped surface, and high surface energy.…”

Abiotic Dehalogenation by Metals

“…A few studies have been conducted with granular iron (43,51,112,116). Arnold and Roberts (7) noted that in the presence of granular zinc, most vicinal polychloroethanes react exclusively via reductive including HCA, both TeCAs, 1,1,2-TCA, and 1,2-dichloroethane (1,2-DCA).…”

“…Scherer et al (112) developed a linear free-energy relationship (LFER) for reactions of chlorinated methanes, ethanes, and ethenes with zero-valent iron by using published reaction rate constants and values. This analysis revealed a direct relationship between faster reduction kinetics and increasing values (Figure 9.3).…”

“…The apparent discrepancy between the conclusions of these two groups of researchers is puzzling, but it may arise in part from the different data sets considered. Arnold and Roberts (8) included only chlorinated ethenes in their correlation, while Scherer et al (112) included chlorinated methanes, ethanes, and ethenes in their analysis.…”