Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 55

ground. As a result, the contaminants are transported towards the anode
or cathode electrode sites by ionic or electrophoretic migration, and elec-
troosmotic advection. The contaminants are then removed at the electrode
sites by one of several different methods: electroplating; adsorption; pre-
cipitation complexation, or pumping.
The electrokinetic treatment is a powerful soil clean-up technique
explored both in the laboratory and the field for the last several decades,
particularly for difficult site conditions. The field applications have fre-
quently demonstrated that the success of the treatment is hinged on site
specific engineering design of the total system, which includes the enhance-
ment of the transport (Lageman, 1993; Yeung et al., 1996; Acar et al., 1997;
Lindgren, 1998). Some of the phenomena experienced with electrokinetic
treatment of soils which, interfere with the steady transport of fluid or
charged particles to an electrode site are: (i) the spatial and transient varia-
tion of soil pH, electric gradient and pore pressures, (ii) surface polarization
effects on particles and electrodes, (iii) other electrochemical effects.
The electrolysis reactions that occur at the electrode sites of the applied
potential can produce substantial changes in pH of the pore solution. The
reactions are oxidation at the anode generating hydrogen (hydronium) ion
and oxygen gas, and reduction at the cathode generating hydroxyl ions and
hydrogen gas, as follows:

Anode: H 2 O ⇔ 2H + + (1/2)O 2 +2e - (Standard potential)E 0 = +1.23 V

Cathode: 2H 2 O + 2e - ⇔ 2OH - +H 2 (Standard potential)E 0 = -0.83 V

The rates at which the hydrogen and hydroxyl ions are produced depend
on the current, as such the lower the current density, the lower the rate. The
hydrogen and the hydroxide ions are transported into the soil by electromi-
gration. The resulting pH fronts influence the adsorption/desorption and
precipitation/dissolution reactions of ionic constituents in the soil pores, as
well as the surface chemistry and electrochemistry of the mineral particles.
The reduced pH is beneficial in desorption of most metals from soil, as well
as dissolution of most metal precipitates. Natural soils with high buffer-
ing capacity and carbonate content, or those under the groundwater table
tend to neutralize the acid front and maintain a higher pH environment
(Pamukcu, 1994). The base front generated at the cathode electrode pen-
etrates the soil a narrow distance creating a zone of high pH, promoting
precipitation of the inorganic species into insoluble hydroxide salts in the
pore space and also their retention on the clay surfaces. . These side effects
may be overcome by continuous adjustment of the pH at the analyte and
the catholyte, and/or by adjusting the current density in the soil.