Kinetics, environmental fate

Larson, R.J. Role of biodegradation kinetics in predicting environmental fate, in Biotransformation and Fate of Chemicals in the Aquatic Environment, Maki, A.W., Dickson, K.L., and Cairns, J., Jr, Eds., American Society of Microbiology, Washington, 1980, pp. 67-86. 

Simple models are used to Identify the dominant fate or transport path of a material near the terrestrial-atmospheric Interface. The models are based on partitioning and fugacity concepts as well as first-order transformation kinetics and second-order transport kinetics. Along with a consideration of the chemical and biological transformations, this approach determines if the material is likely to volatilize rapidly, leach downward, or move up and down in the soil profile in response to precipitation and evapotranspiration. This determination can be useful for preliminary risk assessments or for choosing the appropriate more complete terrestrial and atmospheric models for a study of environmental fate. The models are illustrated using a set of pesticides with widely different behavior patterns. 

Environmental Fate. Sensitized photolysis studies in water and oxidation/reduction studies in both air and water are lacking, as are biodegradation studies in surface and groundwaters. These kinds of studies are important, since they represent the fundamental removal mechanisms available to isophorone in the environment. In addition, the kinetic studies for the atmospheric reactions are important for understanding the significance of a removal mechanism and predicting the reactions that may control the fate of a chemical in the environment. 

Environmental Fate. Available data make it clear that BCME is not likely to endure in the environment. No further studies appear to be required on fate in water or other moist media (food, soil), since the principal fate is rapid hydrolysis. Additional studies on the kinetics of BCME destruction in air by oxidation and hydrolysis would be valuable in refining mathematical models used to calculate levels of BCME in air around a point source. 

TABLE 6.6 Useful Properties of the Chemical, the Medium, and Transport/Kinetics for Assessment of Environmental Fates of Contaminants14,18 53... 

Natural water samples and humic substance solutions were probed for their phototransient behavior. Laser flash kinetic spectroscopy was used to study two transients common to most samples. One transient with a maximum around 720 nm was quenched by decreasing pH and nitrous oxide. It was present In all waters with DOC and had a spectrum which resembled chat of a solvated electron. The signal was linear with laser power. The quantum yield for this transient was measured. Samples with higher ground state absorbance yielded a transient with a maximum at 475 nm Chat was quenched by oxygen. This transient seemed to be a photophysical hybrid with triplet and radical cation character. Additional work done to characterize these transients and predict their environmental fates Is discussed. 

With the increasing emphasis on assessing the environmental fate and effects of chemicals before their potential release into the environment, the kinetics/rates modeling approach is receiving considerable research attention. Most model development work has involved aquatic environments (54, 65, 84, 85, 86, 108). Since water pollution has long been recognized as a serious problem, many of the physical and chemical processes affecting the behavior of chemicals in water have been carefully studied. 

Some environmental fate processes are not usefully modeled as equilibrium problems because the rate of the reaction is more important to quantify than the final composition of the system. Given enough time, a tree that falls on a forest floor will decompose, a pesticide applied to an agricultural field will degrade, and an open keg of beer will go flat. In such cases, the question of interest is not the final state, but how long it takes to get there—days, years, or centuries. In this chapter, only the kinetics of chemical reactions is presented. Kinetics of chemical transfer between phases is not discussed until subsequent chapters because rates of chemical transfer depend on the specific transport characteristics of the media (as well as on the properties of the chemicals themselves). 

James N. Pitts, Jr., is a Research Chemist at the University of California, Irvine, and Professor Emeritus from the University of California, Riverside. He was Professor of Chemistry (1954-1988) and cofounder (1961) and Director of the Statewide Air Pollution Research Center (1970-1988) at the University of California, Riverside. His research has focused on the spectroscopy, kinetics, mechanisms, and photochemistry of species involved in a variety of homogeneous and heterogeneous atmospheric reactions, including those associated with the formation and fate of mutagenic and carcinogenic polycyclic aromatic compounds. He is the author or coauthor of more than 300 research publications and three books Atmospheric Chemistry Fundamentals and Experimental Techniques, Graduate School in the Sciences—Entrance, Survival and Careers, and Photochemistry. He has been coeditor of two series, Advances in Environmental Science and Technology and Advances in Photochemistry. He served on a number of panels in California, the United States, and internationally. These included several National Academy of Science panels and service as Chair of the State of California s Scientific Review Panel for Toxic Air Contaminants and as a member of the Scientific Advisory Committee on Acid Deposition. 

Based on the compounds photolytic behavior in water (see Section 5.3.2.2), direct photolysis in air is expected to be the primary fate process in air. However, no data were available on the vapor-phase photolysis of the compounds that would permit estimation of their half-lives in the atmosphere. If degradation follows simple kinetics, these half-lives are important since they indicate the degree of persistence of a compound in a certain environmental medium. 

Despite the novel positive acquisitions of knowledge from experimental and theoretical studies of gas-phase elemental mercury chemistry there are still large gaps before a complete imderstanding of the fate of mercury in the atmosphere is obtained. It is essential to provide kinetic data and information about formed products. There are some limited studies on the kinetics of gas-phase elemental mercury oxidation on surfaces [68-70]. However, experimental studies on uptake or kinetics of heterogeneous reactions of mercury on various environmentally relevant surfaces such as ice, snow, and aerosols and biomaterials, are needed. 

Since the main tropospheric sink of oxygenated VOCs seems to be their reactions with OH radicals, at least at daytimes, the mechanistic and kinetic information discussed in this work is relevant to fully understand the tropospheric chemistry of such compounds, as well as their subsequent fate. Hopefully, the large amount of experimental and theoretical work that has been revisited here, which has been devoted to chemical reactions of environmental significance, could contribute in some extent to act in the right directions and prevent more damage to our atmosphere. 

The influence of chemical equilibrium and/or kinetics on the progress of chemical reactions often determines the abundance, distribution, and fate of substances in the environment. An understanding of the basic concepts of chemical equilibrium and chemical kinetics, therefore, may help us to explain and predict the environmental concentrations of inorganic and organic species in aqueous systems, whether these species are present naturally or have been introduced by humans. In this chapter we will examine chemical equilibrium. The following chapter considers chemical kinetics or the study of rates of chemical reactions. 

For purposes of simulation of the fate of copper in a realistic environmental situation, the kinetic model just described was imbedded in a previously developed two-dimensional finite element model (11) ... 

Sparks DL, Fendorf SE, Zhang PC, Tang L. (1993). Kinetics and mechanisms of environmentally important reactions on soil colloidal surface. In Migration and Fate of Pollutants in Soils and Subsoils, Vol. 32, NATO ASI Series G (eds. D Petruzzeli, F Helfferich). Berlin, Germany Springer-Verlag, pp. 141-168. 

Sorption reactions at the mineral/water interface significantly affect the mobility, speciation, and bioavailability of trace metal ions in aquatic and soil environments. Therefore, one must precisely understand the kinetics and mechanisms of metal sorption on mineral surfaces to accurately predict the fate of such pollutants in subsurface environments and to facilitate effective environmental remediation procedures. 

Accordingly, to accurately predict the fate, mobility, speciation, and bioavailability of environmentally important metals and semi-metals in terrestrial and water environments, one must understand the kinetics and mechanisms of the reactions. This chapter focuses on nonequilibrium aspects of metal sorption/desorption and the confirmation of reaction mechanisms using in-situ atomic/molecular level spectroscopic and microscopic techniques. 

Since complex environmental conditions vary from site to site, one has to assume an idealized environment. This allows one to make generalizations regarding both the chemical and the environmental properties determining fate. The result of kinetics/rates modeling is a series of predictions of chemical concentrations in each of the environmental media—air, water, soil/sediment, and biota. The final values are a function of the specified chemical input rate. 

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