Chapter 16 - Kinetics and mechanisms of hydrothermal organic reactions

Abstract

The organic compounds dissolved in natural waters primarily originate from the decay of vegetation and animal material, as well as from geothermal sources. The amount of these organic substances broadly depends on the source of the water. As a class, organic substances in natural waters are commonly referred to in terms of the dissolved organic carbon (DOC) based on a chemical analysis. The dissolved organic compounds may become chemically active during the power cycle. This chapter focuses on the reactions of the dissolved organic compounds. Typically, less than 25% of the DOC is in the form of well-characterized compounds. The majority of the DOC is in the form of humic substances, which include the subcategories of humic acid, fulvic acid, and hydrophilic acids. One method of subdividing DOC according to humic, fulvic, and hydrophilic acids is by the method of isolation. Batch and flow reactor methods followed by post-reaction analysis of the products are widely used to study hydrothermal reactions. Three levels of detail can be identified to describe the kinetics of reactions: the mechanistic level, the pathway level, and the global or lumped level. The chapter describes the experimental methods to study hydrothermal reactions. Vibrational spectroscopy (infrared and Raman), nuclear magnetic resonance spectroscopy (NMR), and time resolved UV–Vis spectroscopy provide the greatest opportunity to obtain the kinetics and mechanisms of organic reactions. X-ray absorption fine structure (XAFS) and UV–Vis spectroscopy, and vibrational spectroscopy, are suited for the study of pairing of organic and inorganic ions.