Citation:

Scheckel, K G. AND C A. Impellitteri. Dissolution Processes, Kinetics. Chapter -, D. Hillel (ed.), Encyclopedia of Soils in the Environment, ISBN: 978-0-12-348530-4 . Elsevier Ltd, Oxford, Uk, , p. 400-409, (2004).

Impact/Purpose:

To examine the dissolution kinetics as a function of the minerals and compounds present in soils and to evaluate the fundamental rates of dissolution when pondering the comprehensive health and sustainability of the natural environment.

Description:

Chemistry by its very nature is concerned with change. There are simple but significant interactions between air, water, and minerals that impact our natural environment. Minerals with well-defined structure are converted by various environmental chemical reactions into their elemental building blocks with, perhaps, differing chemical properties relative to the original crystal configuration. The influence of natural phenomena may cause minerals to dissolve in aqueous solutions, thus a solution of atoms is eventually formed, succumbing to the fundamental natural law of element-cycling. Further, these reactions are rarely at equilibrium. They proceed at varying kinetic rates as any dissolved material in solution may be removed (e.g., mineral recrystallization) or added by further mineral dissolution. Recent studies have shown that mineral-like, divalent metal surface precipitates exhibit similar dissolution behaviors to clay and oxide minerals. The examination of mineral dissolution can become even more complex when interaction of surfaces with microorganisms and charged compounds are considered that may induce reductive dissolution of redox-sensitive materials. The study of kinetics can be defined broadly as the rate of change of concentration of reactants in a chemical reaction. The rates are affected by both physical (e.g., diffusion of reacting species) and chemical processes. The kinetics involved in dissolution are often ignored in environmental studies by eliminating time as a variable and assuming a state of equilibrium or 'pseudo' equilibrium. For example, the myriad chemical extractions utilized to assess the potential mobility of metals do so in a set time period. Generally, the rigorousness of a particular extraction acts as a surrogate for time. However, the assumptions employed may be more suitable for some soils than others because of the high variation in dissolution kinetics as a function of the minerals and compounds present. In the short term, (e.g., time of a typical soil chemical extraction) a soil containing relatively small concentrations of weakly sorbed Pb may be assessed as a higher risk than a soil containing a high concentration of galena (PbS). In the long term, as the kinetics of dissolution approach equilibrium, the actual risk may be much higher for galena soil. Thus, understanding the fundamental rates of dissolution is necessary when pondering the comprehensive health and sustainability of the natural environment.

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