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Sparks, D.L. 1999. Kinetics and mechanisms of soil chemical reactions. p. B-123-167. In M.E. Sumner (ed) Handbook of Soil Science, CRC Press, Boca Raton, FL.

Handbook of Soil Science

Kinetics and Mechanisms of Soil Chemical Reactions

Donald L. Sparks

Introduction

Since its inception in the mid 1850s, soil chemistry has focused on the macroscopic, equilibrium aspects of soil chemical reactions and processes. From these studies, much was learned about important soil chemical processes including sorption, desorption, precipitation, Complexation, dissolution and oxidation/reduction. However, such investigations do not convey information on reaction rates or reaction mechanisms. However, such investigations do not convey information on reaction rates or reaction mechanisms. In the past two decades, as concerns and interests about soil and water quality have increased, soil and environmental chemists, environmental and chemical engineers and geochemists have increasingly realized that reactions in subsurface environments are time dependent. Thus, to accurately predict the fate, mobility, speciation, and bioavailability of environmentally important plant nutrients, trace elements, radionuclides, and organic chemicals in soils, one must understand the kinetics and mechanisms of the reactions.

While major progress has been made in better understanding the kinetics of soil chemical processes, much uncertainty remains. In part, this is due to the complex, heterogeneous nature of natural materials such as soils. However, with development of kinetic techniques that can be used to measure a wide range of time scales, time-dependent models that can describe both chemical reaction and mass transfer processes, and the employment of state-of-the-art in situ spectroscopic and microscopic surface techniques in combination with rate studies, major advances are being made in understanding the kinetics and mechanisms of soil chemical reactions. Arguably, this will be a major leitmotif in soil chemistry research for decades to come.

In this review, the application of chemical kinetics to heterogeneous systems such as soils and soil components (clay minerals, organic matter, and humic substances), with emphasis on sorption/release processes will be discussed. A critical review of kinetic models that can be used to describe reaction rates on heterogeneous surfaces will be covered. The review will also present discussions on the rates of important soil chemical reactions and processes including inorganic and organic sorption/desorption, dissolution and redox. For additional details on these topics and other aspects of kinetics of soil chemical and geochemical processes, the reader should consult a number of recent books and monographs (Sparks, 1989; Sparks and Suarez, 1991; Stumm, 1992; Schwarzenbach et al., 1993; Sposito, 1994; Sparks, 1995).

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