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Sparks, D.L. 1999. Kinetics and mechanisms of chemical reactions at the soil mineral/water interface. In. D.L. Sparks (ed). Soil Physical Chemistry, 2nd edition. CRC Press, Boca Raton, FL. p. 135-192.

Soil Physical Chemistry, Second Edition

Chapter 4

Kinetics and Mechanisms of Chemical Reactions at the Soil Mineral/Water Interface.

Donald L. Sparks

I. INTRODUCTION

Without question, one of the important paradigms in our society is preservation of the environment. Worldwide, concerns have been voiced about numerous soil and water contaminants. These include plant nutrients (e.g., nitrate and phosphate), heavy metal, radionuclides, pesticides, and other organic chemicals.

The reactions that these contaminants undergo with natural particles, such as sediments and soils, involving sorption, desorption, precipitation, complexation, redox, and dissolution phenomena, are critical in determining their fate and mobility in the subsurface environment.

Much of the research on migration and retention of contaminants on natural materials has been studied from a macroscopic, equilibrium approach. The focus of many of these investigations has been on the determination of distribution coefficients (determined primarily on a 24-hour basis), and the use of equilibrium-based models such as the Freundlich, Langmuir, and the various surface complexation models, e.g., constant-capacitance and triple-layer, to determine numerous sorption parameters, information on the physical description of the electric double layer, and data conformance over a wide range of experimental conditions such as varying pH and ionic strength. While the surface complexation models are predicated on molecular descriptions of the electric double layer, equilibrium-derived data are employed and, thus, no direct molecular information is provided.

The above criticism of equilibrium approaches is not meant to imply that they are not useful, since they provide important data on the final state of a reaction. However, they provide no information on reaction rates or mechanisms. Moreover, such equilibrium studies are usually not relevant to field settings, since reactions involving subsurface materials are seldom, if ever, at equilibrium. To understand the rates of chemical reactions on particle surfaces, one must study the kinetics of the reactions. Kinetic studies can assist in revealing reaction mechanisms. Of course, to determine mechanisms directly, one must use microscopic and spectroscopic surface techniques. Ideally, one should follow reaction rates microscopically and/or spectroscopically and couple these with macroscopically observed processes. Such approaches will be discussed later. In short, time-dependent reactions are important factors in controlling the fate and transport of contaminants in the subsurface environment.

Early studies by Way on ion exchange in soils showed that reaction rates were often instantaneous. Similar conclusions were reached by Gedroiz and Hissink. The finding that ion-exchange kinetics were diffusion-controlled was discovered by Boyd et al. In their seminal paper they also elucidated rate-limiting steps for ion exchange. Kelley correctly hypothesized that rates of ion exchange should be highly dependent on the adsorbent. For example, reaction rates on kaolinite, which has only external surface sites, should be higher than on vermiculite, which has planar and edge external as well as internal sites. Reactions on internal sites, depending on their geometry, could be quite slow. Unfortunately, Kelley's astute observations went unnoticed and little research on reaction rates on soils and soil components appeared over the next three decades. There were notable contributions by Helfferich on ion-exchange kinetics and by Mortland and coworkers land Scott and coworkers on the kinetics of potassium release from vermiculites and mica.

In the late 1970s, and certainly in the 1980s and 1990s, the kinetics of environmentally important reactions at the soil mineral/water interface has become a major leitmotif in the soil and environmental sciences and in environmental engineering. This intense interest is in large part due to the recognition that reactions in natural settings are usually time dependent and thus, to predict accurately the fate of contaminants in the subsurface environment, a knowledge of the reaction kinetics is imperative, While major advances have been made in understanding time-dependent reactions on natural materials such as soils and sediments, there are still many unknowns and needs that are complicated by the complex, heterogeneous nature of natural materials. Research needs include models that accurately describe both chemical kinetics and transport processes in multiple- site,heterogeneous systems; better kinetic methods; more extensive studies on the effect of residence time ("aging") on contaminant retention/release; and mechanistic studies that employ time-resolved in situ microscopic and spectroscopic techniques.

In this chapter, I shall discuss 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. A critical review of kinetic models that can be used to describe reaction rates on heterogeneous surfaces will be covered. The chapter will also cover the kinetics of important inorganic and organic sorption/desorption and dissolution reactions at the soil mineral/water interface. Additionally, there are discussions on the use of in situ spectroscopic and microscopic techniques to confirm reaction mechanisms at the soil mineral/water interface. 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.

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