Kinetics, geochemical application

Geochemical kinetics can be viewed as applications of chemical kinetics to Earth sciences. Geochemists have borrowed many theories and concepts from chemists. Although fundamentally similar to chemical kinetics, geochemical kinetics distinguishes itself from chemical kinetics in at least the following ways ... 

There is a general lack of kinetic data for critical environmental and geochemical processes. Additionally, despite recent advances in chemical kinetics, the applicability of laboratory kinetic data to field situations is still uncertain. There is a large discrepancy between laboratory and field derived rate constants (Lasaga,... 

Palandri J. L., Kharaka Y. K. (2004) A compilation of rate parameters of water-mineral interaction kinetics for application to geochemical modeling. U.S. Geological Survey Water-Resources Investigations Report 04-1068. New York, 64 p. 

This chapter provides a general discussion of kinetics versus thermodynamics, chemical kinetics versus geochemical kinetics, and an overview of the basics of various kinetic processes and applications. Subsequent chapters will provide in-depth development of theories and applications of specific subjects. The purpose of the overview in this chapter is to provide the big picture of the whole field before in-depth exploration of the topics. Furthermore, this chapter is a standalone chapter that may be used in a general geochemistry course to introduce kinetics to students. 

Geochemical kinetics is stiU in its infancy, and much research is necessary. One task is the accumulation of kinetic data, such as experimental determination of reaction rate laws and rate coefficients for homogeneous reactions, diffusion coefficients of various components in various phases under various conditions (temperature, pressure, fluid compositions, and phase compositions), interface reaction rates as a function of supersaturation, crystal growth and dissolution rates, and bubble growth and dissolution rates. These data are critical to geological applications of kinetics. Data collection requires increasingly more sophisticated experimental apparatus and analytical instruments, and often new progresses arise from new instrumentation or methods. 

Application of the collected thermodynamic data to model the oxidative alteration pathway of U02 under repositoiy conditions by using the PHREEQC code (Parkhurst Appelo 1999) is given in Fig. 1 la and b. Once the thermodynamic framework is set for the geochemical evolution of the repositoiy system, we have to take into consideration that for many of the processes involved, there will be some kinetic constraints. This is illustrated by Table 2, where a comparison of the expected lifetime for some of the phases expected in the repositoiy system is made. 

A major portion of the studies on calcium carbonate reaction kinetics has been done in seawater because of the many significant geochemical problems related to this system. Morse and Berner (1979) summarized the work on carbonate dissolution kinetics in seawater and their application to the oceanic carbonate system. The only major seawater component in addition to Mg2+ that has been identified as a dissolution inhibitor is SO42- (Sjoberg, 1978 Mucci et al., 1989). Sjoberg s studies of other major and minor components (Sr2+, H3BO3, F-) showed no measurable influence on dissolution rates. Morse and Berner (1979) and Sjoberg (1978) found that for near-equilibrium dissolution in phosphate-free seawater, the dissolution rate could be described as ... 

Overall, geochemical computer models can be extremely useful in the description of chemical equilibria occurring in the aquatic environment. In some cases, predictions about reaction kinetics and transport of species can also be made. The application of geochemical models is not limited to natural aquatic systems but has been usefully extended to predict the eflfectiveness of certain remediation strategies in the treatment of waters emanating from contaminated sites." ... 

Application of Geochemical Kinetic Data to Groundwater Systems... 

Salvage K. M. and Yeh G.-T. (1998) Development and application of a numerical model of kinetic and equilibrium microbiological and geochemical reactions (BIOKEMOD). J. Hydrol. 209, 27-52. 

Following are some rate studies and models of geochemical interest. They have been chosen to illustrate the concepts presented in this chapter and to suggest where we are in the development and application of kinetic principles to low-temperature water/rock systems. 

Langmuir, D., and J. Mahoney. 1985. Chemical equilibrium and kinetics of geochemical processes in ground water studies. In Practical applications of ground water geochemistry, Proc. 1st Canadian/American con/, on hydrogeology, ed B. Hitchon and E. I. Wallick, pp. 69-95. Worthington, OH Natl. Water Well Assoc. 

Mackenzie A.S., Beaumont C., McKenzie D.P. (1984) Estimation of the kinetics of geochemical reactions with geophysical models of sedimentary basins and applications. Org. Geochem. 6, 875—84. 

Watson EB, Cherniak DJ (1997) Oxygen diffusion in zircon. Earth Planet Sci Lett 148 527-544 Wendlandt RW (1991) Oxygen diffusion in basalt and andesite melts Experimental results and discussion of chemical versus tracer diffusion. Contrib Mineral Petrol 108 463-471 West AR (1984) Solid State Chemistry and Its Applications. John Wiley and Sons, New York Whipple RTP (1954) Concentration contours in grain boundary diffusion. Phil Mag 45 1225-1236 White AF, Peterson MI (1990) Role of reactive-surface area characterization in geochemical kinetic models. In Melchior DC, Bassett RL (eds) Chemical Modeling of Aqueous Systems. II. Am Chem Soc Symp 416 461-475... 

Major topics include rate equations, reactor theory, transition state theory, surface reactivity, advective and diffusive transport, aggregation kinetics, nucleation kinetics, and solid-solid transformation rates. The theoretical basis and mathematical derivation of each model is presented in detail and illustrated with worked examples from real-world applications to geochemical problems. The book is also supported by online resources self-study problems put students new learning into practice and spreadsheets provide the full data used in figures and examples, enabling students to manipulate the data for themselves. 

Apart from the problem of lack of data, especially kinetic data, this situation probably constitutes the most serious problem in the application of thermodynamics to geochemical problems involving hydrothermal solutions. The development of an algorithm which is more accurate than the B-dot method as presently used, but which could still be used for any species at any T and P would be a significant advance in geochemical modeling practice. 

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