Many chemical reactions imply the transfer of electrons from one chemical species to another. These reactions are called redox reactions and they are usually rather slow. In soil and water, redox reactions involve hydrogen ions and are thus greatly pH dependent. The most important redox reactions involve oxygen, carbon, nitrogen, sulphur, manganese and iron. In polluted soils, arsenic and mercury can also participate.
The redox potential, and changes thereof, play a crucial role in the behaviour of metals in soils. For instance, iron oxides are formed at high redox potentials. Iron oxides and hydroxides are capable of adsorbing heavy metals onto their surfaces, which will greatly reduce the mobility of the heavy metals. When the redox potential is lowered, the iron oxides dissolve and the adsorbed heavy metals are released and will be available for leaching further down the soil profile. Many redox reactions in nature are speeded up by certain bacteria, however. The bacteria utilise the energy released from redox reactions (Berggren Kleja et al., 2006).
Reactions Between Acids and Bases (Proton Exchange)
The amount of protons (H+) in solution greatly influences most chemical reactions. Proton transfer reactions are usually very fast. According to the Brensted definition, protons are provided by an acid and captured by a base. To each acid Ac there is a corresponding base Ba:
Aci Bai + H+
The acid and the corresponding base constitute an acid/base couple (Ac1 /Ba1).
In most natural waters, the pH lies within the range from 5 to 8. All the substances dissolved into water (gases, mineral and organic compounds) contribute to the acid-base equilibrium of water. All components of the carbonate system make a major contribution to the acid neutralizing capacity (called alkalinity) of the water and to its base neutralizing capacity (acidity). The buffering capacity of water (the ability of the water to maintain its pH despite any addition of H+ or OH-) is also largely determined by the carbonate system. However, dissolved silicates, ammonia, organic bases, sulphides and phosphates also contribute to the alkalinity. In like manner, non-carbonic acids, polyvalent metal ions and organic acids contribute to the acidity.
Rainwater often contains strong acids originating in atmospheric pollutants (dissolution of gases leading to HCl, HNO3, H2SO4). Acid rain may increase the heavy — metal solubility in soils. The pH effect of strong acids on soil and water will depend on the buffering capacity of the soil or water, however. Oxidation reactions lead to a decrease in pH whereas reduction tends to increase the pH.
