(GC5-2002)Chemical Fate of Contaminants in the Environment: Chlorinated Hydrocarbons in the Groundwater
EMSL Project ID
2403
Abstract
Chlorinated hydrocarbons (CHCs) are the most common contaminant found at hazardous waste sites and are the most prevalent contaminants on Department of Energy (DOE) weapons production sites. Many of the chlorinated hydrocarbons are either known or suspected carcinogens and thus pose health risks to the public and/or site workers. Chlorinated hydrocarbons, unlike simple hydrocarbons, are resistant to biodegradation, but can degrade by abiotic processes such as hydrolysis, nucleophilic substitution, and dehydrochlorination. Unfortunately, few studies of the reactions of chlorinated hydrocarbons have been reported in the literature, and disagreement still exist about the mechanisms and rates of many of the key reactions. We propose to model the reactions involved in the degradation of chlorinated hydrocarbons in the groundwater and the formation of toxic chlorinated compounds such as dioxin. The goals of the research proposed here are fourfold: ? Development of a computational approach that will allow reaction pathways and rate constants to be accurately calculated ? Development of more approximate approaches, evaluated against the more accurate approach, which will lay the groundwork for exploratory studies of more complex CHCs ? Application of these approaches to study the degradation pathways of CHCs in aqueous liquids ? Application of the more approximate approaches to study mechanism of forming complex chlorinated hydrocarbon polychlorinated benzene compounds and dioxins. Over the past three years significant progress has been made on developing a hierarchical computational approach that builds up from high level ab initio calculations on reaction in isolation to more approximate calculations in solvated clusters and bulk solvent. The primary focus of this development effort was high-level electronic structure calculations on gas-phase and microsolvated reactions. In the research program presented here, we propose to continue the development effort, with a focus on evaluation of approximate methods to include solvation effects. Layered electronic structure methods (e.g., ONIOM) have been tested against benchmark calculations and will provide the basis for evaluating more approximate hybrid quantum mechanical/molecular mechanics methods (e.g., QM/MM and IMOMM). The QM/MM methods treat solvent molecules explicitly and can be used to evaluate approaches that treat the solvent by a continuum model (e.g., dielectric continuum models). A major focus of this work will be the development of continuum solvation models, which are based upon the solvation models of Truhlar and Cramer, and which are appropriate for density functional calculations. Previous work was primarily focused on the initial elementary steps in reactions of OH - with chlorinated methane molecules, CHn Cl(4-n) , and studies of the reactions of OH - with chlorinate ethylene molecules, C2 Hn Cl(4-n), were initiated. Future studies will extend calculations to study reactions that the products of these initial reactions can undergo. For example, the nucleophilic substitution reaction of OH - with CH2 Cl2 forms a chlorinated methanol molecule, which can undergo HCl elimination to form formaldehyde. These types of reactions will be the subjects of our future studies. In addition, we will employ more approximate, and less costly, computational methods to study reaction involving more complex chlorinated compounds such as polychlorinated benzene molecules and dioxins.
Project Details
Project type
Capability Research
Start Date
2001-12-04
End Date
2004-11-15
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Analytic Potential Energy Functions for Simulating Aluminum Nanoparticles
Liu J, P Zhang, K Morokuma, and RD Sharma. 2005. "A New Mechanism for the Production of Highly Vibrationally Excited OH in the Mesosphere: An AB Initio Study of the Reactions of O-2(A (3)Sigma(+)(u) and A ' (3)Delta(u))+H ." Journal of Chemical Physics 122(10):104315.
Re S, and K Morokuma. 2001. "ONIOM Study of Chemical Reactions in Microsolvation Clusters: (H2O)(n)CH3Cl+OH-(H2O)(m) (n+m = 1 and 2)." Journal of Physical Chemistry A 105(30):7185-7197.
Schultz NE, and DG Truhlar. 2005. "New Effective Core Method (Effective Core Potential and Valence Basis Set) for Al Clusters and Nanoparticles and Heteronuclear Al-Containing Molecules." Journal of Chemical Theory and Computation 1(1):41-53.