Many redox reactions in the natural environment proceed too slowly to approach their equilibrium state, absent the action of catalysts or enzymes. In this chapter, we consider how to trace multicomponent reaction paths in which the progress of redox reactions is described by kinetic rate laws. We frame our discussion in terms of three example calculations. In the first example, ferrous iron both catalyzes and provides reducing power to convert uranyl to insoluble uranium hydroxide. The autocatalysis of manganese, in which the reaction product operates as the catalyst that promotes the reaction, serves as the second example, and the microbial degradation of phenol makes up the final illustration of the method.

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.