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HYDROLYSIS
Citation:
Wolfe, N L. AND P M. Jeffers. HYDROLYSIS. Chapter 13, R.S. Boethling and D. Mackay (ed.), Handbook of Property Estimation Methods for Chemicals: Environmental and Health Sciences. Lewis Publishers, Boca Raton, FL, , 311-333, (2000).
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:
Hydrolytic processes provide the baseline loss rate for any chemical in an aqueous envi- ronment. Although various hydrolytic pathways account for significant degradation of certain classes of organic chemicals, other organic structures are completely inert. Strictly speaking, hydrolysis should involve only the reactant species water provides - that is, H+, OH-, and H20 - but the complete picture includes analogous reactions and thus the equivalent effects of other chemical species present in the local environment, such as SH- in anaerobic bogs, Cl- in sea water, and various ions in laboratory buffer solutions.
Methods to predict the hydrolysis rates of organic compounds for use in the environmental assessment of pollutants have not advanced significantly since the first edition of the Lyman Handbook (Lyman et al., 1982). Two approaches have been used extensively to obtain estimates of hydrolytic rate constants for use in environmental systems. The first and potentially more precise method is to apply quantitative structure/activity relationships (QSARs). To develop such predictive methods, one needs a set of rate constants for a series of compounds that have systematic variations in structure and a database of molecular descriptors related to the substituents on the reactant molecule. The second and more widely used method is to compare the target compound with an analogous compound or compounds containing similar functional groups and structure, to obtain a less quantitative estimate of the rate constant.
Predictive methods can be applied for assessing hydrolysis for simple one-step reactions where the product distribution is known. Generally, however, pathways are known only for simple molecules. Often, for environmental studies, the investigator is interested in not only the parent compound but also the intermediates and products. Therefore, estimation methods may be required for several reaction pathways. Some preliminary examples of hydrolysis reactions illustrate the very wide range of reactivity of organic compounds. For example, triesters of phosphoric acid hydrolyze in near-neutral solution at ambient temperatures with half-lives ranging from several days to several years (Wolfe, 1980), whereas the halogenated alkanes pentachloroethane, carbon tetrachloride, and hexachloroethane have "environmental" (pH = 7, 25 degrees C) half-lives of about 2 hr, 50 yr, and 1000 millennia, respectively (Mabey and Mill, 1978; Jeffers et al., 1989). On the other hand, pure hydrocarbons from methane through the PAHs are not hydrolyzed under any circumstances that are environmentally relevant.
Hydrolysis can explain the attenuation of contaminant plumes in aquifers where the ratio of rate constant to flow rate is sufficiently high. Thus 1,1,1-trichloroethane (TCA) has been observed to disappear from a mixed halocarbon plume over time, while trichloroethene and its biodegradation product 1,2-dichloroethene persist. The hydrolytic loss of organophosphate pesticides in sea water, as determined from both laboratory and field studies, suggests that these compounds will not be long-term contaminants despite runoff into streams and, eventually, the sea (Cotham and Bidleman, 1989). The oceans also can provide a major sink for atmospheric species ranging from carbon tetrachloride to methyl bromide. Loss of methyl bromide in the oceans by a combination of hydrolysis and Cl- for Br- exchange constitutes a significant contribution to the total degradation and is a key factor in modeling atmospheric concentrations and balance schemes. It is therefore an important part of the assessment of stratospheric ozone depletion potential.