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PREDICTION OF PHYSICOCHEMICAL PROCESSES FOR ENVIRONMENTAL MODELING BY COMPUTER
Citation:
Hilal, S H. AND L. A. Carreira. PREDICTION OF PHYSICOCHEMICAL PROCESSES FOR ENVIRONMENTAL MODELING BY COMPUTER. Presented at 226th American Chemical Society National Meeting, New York, NY, September 7-11, 2003.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
The major differences among behavioral profiles of molecules in the environment are attributable to their physicochemical properties. For most chemicals, only fragmentary knowledge exists about those properties that determine each compound's environmental fate. A chemical-by-chemical measurement of the required properties is not practical because of expense and because trained technicians and adequate facilities are not available for measurement efforts involving thousands of chemicals. In fact, physical and chemical properties have only been measured for about 1 percent of the approximately 70,000 industrial chemicals listed by the U.S. Environmental Protection Agency's Office of Prevention, Pesticides and Toxic Substances. Hence, the need for physical and chemical constants of chemical compounds has greatly accelerated both in industry and government as assessments are made of potential pollutant exposure and risk. Although a wide variety of approaches are commonly used in regulatory exposure and risk calculations, knowledge of the relevant chemistry of the compound in question is critical to any assessment scenario. For volatilization, sorption and other physical processes, considerable success has been achieved in not only phenomenological process modeling but also a priori estimation of requisite chemical parameters, such as solubilities and Henry's law constants. Granted that considerable progress has been made in process elucidation and modeling for chemical processes such as photolysis and hydrolysis, reliable estimates of the related fundamental thermodynamic and physicochemical properties (i.e., rate and equilibrium constants, distribution coefficient, solubility in water, etc.) have been achieved for only a limited number of molecular structures. The values of these latter parameters, in most instances, must be derived from measurements or from the expert judgment of specialists in that particular area of chemistry. Mathematical models for predicting the transport and fate of pollutants in the environment require reactivity parameter values that is, the physical and chemical constants that govern reactivity. Although empirical structure activity relationships have been developed that allow estimation of some constants, such relationships are generally valid only within limited families of chemicals. A computer program has been under development for more than 12 years that predicts a large number of chemical reactivity parameters and physical properties for a wide range of organic molecules strictly from molecular structure. This prototype computer program called SPARC (SPARC Performs Automated Reasoning in Chemistry) uses computational algorithms based on fundamental chemical structure theory to estimate a variety of reactivity parameters. This capability crosses chemical family boundaries to cover a broad range of organic compounds.