1.1 Electro-Kinetic Phenomena
The electro-kinetic phenomenon of soil was first discovered by Reuss in 1807 at Moscow State University. Reuss reported his discovery on the conference of Society of Competition of Medical and Physical Sciences and then a publication of Notice on a New Effect of Galvanic Electricity was made on the Memoirs of the Imperial Society of Naturalists of Moscow in 1809.
There are five major electro-kinetic phenomena in the soil, namely streaming potential, migration potential, electro-osmosis, ion migration and electrophoresis.
(1) Streaming potential
If there is seepage flow under hydraulic gradient, electric potential in the soil will be increasing along the flow direction. The electric potential generated by seepage flow is called streaming potential. The streaming potential in clayed soil could be tens of millivolts. The creation of streaming potential is due to the migration of ions along the seepage flow. As we know that most clay particles are in negative charge, so to maintain electric neutrality, there must be more cation than anion in the pore water. Therefore, there are more positive charges migrate with the seepage flow, which builds up the electric potential along the flow direction. Another microscopic interpretation could be that streaming potential is a result of offset of electrical double layer along the seepage flow direction.
(2) Migration potential
Migration potential is also caused by migration of electrical charges. Instead of ions, it is caused by the migration of soil particles under non-electrical effect (e.g. settlement of particles under gravity). Most soil particles are in negative charge, so electric potential is decreasing along the particle migration direction. Similarly, the microscopic interpretation of migration potential could be linked to the concept of double layer. The migration potential is created because that the viscosity of water holds back the movement of cation in diffusion layer.
Electro-osmosis refers to the phenomenon that pore water of soil moves from anode to cathode under a DC electric field. Electro-osmotic flow is caused by the dragging effect of ions to the surrounding pore water. There is more cation than anion in the water, so net effect of water migration is from anode to cathode.
(4) Ion migration
Ions or ion complexes will migrate to the corresponding electrodes according to their electrical charges. Lageman et al (1989)  suggested that average mobility of ions in soil pore water is 5×10-8m2/v·s.
Electrophoresis is the migration of soil particles under DC electric field. Most soil particles are in negative charge, so they will migrate towards anode. According to van Olphen (1977) , the electrophoresis mobility of soil particles is 1×10-8~3×10-8 m2/v·s, while according to Lageman et al (1989)  the range is 1×10-10~3×10-10 m2/v·s.