Selected Geochemical Processes - Contaminant Geochemistry - page 42

Fig. 2.3 Rate-limiting steps in mineral dissolution: a transport-controlled, b surface reaction-
controlled, and c mixed transport and surface reaction control. Concentration (C) versus distance
(r) from a crystal surface for three rate-controlling processes, where C eq is the saturation
concentration and C ? is the concentration in an infinitely diluted solution. Reprinted from Sparks
( 1988 ). Copyright 2005 with permission of Elsevier

2.2.2 Redox Processes

Redox processes in the subsurface involve the transfer of electrons among the
constituents of aqueous, gaseous, and solid phases. As a result, changes in the
oxidation state of the reactants and products occur. Oxidation is the half reaction
where electrons are lost or removed from a species, while reduction is the half
reaction where electrons are gained or added to a species. Accordingly, in a redox
reaction, an oxidation agent (or oxidant) is an electron acceptor, and a reducing
agent (or reductant) is an electron donor.
All oxidation reactions are coupled to reduction reactions. In many cases, redox
reactions can also involve or be affected by changes in the surrounding environ-
ment, such as changes in the pH or temperature (i.e., endothermic or exothermic
reactions). Many elements in the subsurface can exist in various oxidation states;
some examples include elements like carbon, nitrogen, oxygen, sulfur, iron, cobalt,
vanadium, and nickel.
The great importance of redox reactions and redox potentials in biogeochemical
processes and their effect on the subsurface system is well established. In general,
the redox potential pE (or Eh) and pH are considered the principal variables
controlling geochemical reactions in geological and aquatic environments; many
of the processes that are discussed in the next chapters involve redox reactions.
The redox potential, Eh, is also commonly given as pE, the measure of electron
activity analogous to pH. By convention, half reactions are written in terms of
reduction; for example,

aA þ bB þ e cC þ dD : ð 2 : 34 Þ