Geochemical Modeling of Reaction Paths and Geochemical Reaction Networks

Abstract

From the discussions in previous chapters, we can see that in order to understand the kinetics of dissolution and precipitation reactions, we really need to understand geochemical reaction networks. The previous chapters have depicted a complex picture of the details of chemical reactions and kinetics. The complexity comes from the nature of chemical kinetics. Unlike thermodynamics, which describes one state versus another, independent of the reaction paths, the kinetics of chemical reactions is path dependent. Lasaga (1998) cited Benson (1960) who described the time- and path-dependent nature of reaction kinetics “ a body of water on top of a hill may be described thermodynamically in terms of its composition, pressure, and temperature. At a later time, this same body of water may find its way to a lake below. The thermodynamic description of the body of water in the lake is again well defined. However, if we try to describe the transition—the water in process of flowing from the hilltop—we see that it may depend on almost innumerable factors: on the outlets, on the contour of the hillside, on the structural stability of the contour, and on the numerous subterranean channels through the hillside that may exist and permit seepage. ” What this familiar quote highlights is the path dependent nature of chemical kinetics, which helps to weave together a bewildering variety of geochemical reaction networks in many geologic systems. How can we handle this complexity and apply chemical kinetics to the geological systems? It is certainly impossible to conduct laboratory experiments to determine every reaction path and explore every type of geochemical reaction network. It appears that geochemical modeling, if standing on solid building blocks of theory and experimental data, can play a critical role in exploring the ranges of behaviors of geochemical reaction networks and extrapolate laboratory experimental …