Chemists commonly find need to trace the rates at which aqueous species form complexes with other species in solution, or with reactive sites on mineral surfaces, and conversely how rapidly such complexes break apart. This chapter shows how kinetic rate laws applied to association and dissociation reactions can be incorporated into multicomponent chemical reaction models and provides a fully worked calculation, using aluminum fluoride complexing as an example.

Natural waters near Earth’s surface commonly exist far from redox equilibrium and hence hold a thermodynamic drive for the oxidation of some aqueous species at the expense of others, which are reduced. The rates at which such oxidation and reduction reactions occur in the natural environment are described by kinetic laws, which may account for heterogeneous catalysis or promotion by enzymes. This chapter shows how to incorporate redox kinetics into multicomponent chemical reaction models and gives a fully worked example of how such models can be applied.

The kinetics of microbially catalyzed reactions are of special interest because of the control the reactions exert of the redox state of laboratory experiments and the natural environment. A general description of microbial kinetics must address the requirement of thermodynamic consistency, so the kinetic laws apply equally well far from chemical equilibrium, and close to it. This chapter shows how to formulate thermodynamically consistent rate laws for microbial respiration and fermentation, the process of incorporating such laws into multicomponent chemical reaction models, and a fully worked example demonstrating how such models behave.

The movement of gas species across the air–water interface is a central aspect of biogeochemical cycling and plays a critical role in controlling not only the composition of the atmosphere, but the chemistry of aquatic and marine systems. This chapter shows how kinetic rate laws describing the transfer of gas species into and out of aqueous solution can be integrated into multicomponent chemical reaction models and shows a fully worked calculation, using carbon dioxide efflux from a biologically active lake as an example.

Surface complexation is the process by which aqueous species react chemically with specific sites at the aqueous–solid interface to form surface complexes. This chapter describes in detail electrostatic double-layer theory and how it can be integrated into multicomponent equilibrium models. A worked example demonstrates how the theory might be applied in the study of natural waters.

The sorption of aqueous species onto solid surfaces exerts an important control on the mobility and bioavailability of dissolved mass in the natural environment. This chapter shows how sorption reactions can be incorporated into multicomponent speciation models using distribution coefficients, Freundlich isotherms, Langmuir isotherms, and ion exchange theory.