*4.3.1 Thermodynamics*

The idea is to couple two electrochemical half-reactions, one half-reduction reaction and one half-oxidation reaction, having the former a higher standard reduction potential than the latter. As a requirement, since this wastewater treatment must occur without any energy input, the reaction must evolve spontaneously. Such a condition is presented in Eq. (25) [79].

$$E\_{\rm RED} - E\_{\rm OX} > 0 \tag{24}$$

Using Nernst equation, Eq. (25) would be of use to assess a first analysis of the impact of varying activities of different ions, partial pressures, or temperature on the spontaneity of the process can be assessed.

*Fundamentals and Practical Aspects of Acid Mine Drainage Treatment: An Overview from Mine… DOI: http://dx.doi.org/10.5772/intechopen.104507*

### *4.3.2 Kinetics*

Electrochemical kinetics of spontaneous redox reactions are commonly studied in terms of the mixed potential theory and the corresponding current density. The mixed potential which is not a thermodynamic parameter is obtained from equalising the anodic and cathodic reactions [Eq. (26)]

$$I\_{anodic} = |I\_{cathodic}|\tag{25}$$

In a complex system, there could be several reduction reactions and oxidation reactions occurring simultaneously in different locations within the system, therefore several mixed potentials may be installed. The open rest potentials, in this case, will attempt to follow such mixed potentials and depending on the conductivity of the species formed at the solid interface and at aqueous solution bulk such tracking down will be faster or slower [80].
