Electrodialytic remediation (EDR) is a variation of electrokinetic remediation (EKR), in which ion-exchange membranes are applied as barriers between soil and electrolyte solutions. Under the influence of a direct current (DC) electric field, transport of free ions, soil solution and small charged particles is induced into the soil. This transport may be utilized for removal of contaminants. The fundamental principles of the transport processes are described below in order to establish an understanding of the remediation process prior to the discussion of potential applications of microbial products.
2.1 ELECTROMIGRATION
Electromigration refers to the movement of individual ions within the soil solution driven by the electrical potential gradient. The flux of an ionic specie caused by electromigration is described by equation (1) (Acar and Alshawabkeh, 1993):
Jjm = -uj* cj ∇U (1)
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In this equation cj is the concentration of specie j, ∇U is the gradient of the electrical potential, and u*j is the effective ionic mobility. u*j is related to the ionic mobility in free solution according to equation (2):
uj* = u x
j e
(2)
xe, the tortuousity, accounts for the longer traveling distance in soil compared to free solution. The ionic mobility can be related to the diffusion coefficient through the Einstein relation (Atkins, 1994):
uj = D z F RT
j j (3)
Here Dj is the diffusion coefficient in free solution, and zj the valence if the ion. Dj
can be calculated according to the Stokes-Einstein equation:
Dj =
j
kT
6 a (4)
η is the viscosity of the fluid, and aj is the radius of the particle in question. It is important to notice that for ions, aj is the hydrated ionic radius as opposed to the free ionic radius. Bringing together (3) and (4), equation (5) is obtained:
uj = j
j
z e
6 a (5)
It appears that the electromigration of a specific ion relies upon the ion-concentration and -valence, as well as the size of the hydrated ion, the viscosity of the fluid and the electric potential.
Because diffusion coefficients for hydrated ions are more readily available than their hydrated radius itself, equation (3) is more useful for practical calculation of the ionic mobility. The order of uj [10-8 m2/(V s)] for chosen ions is given below (Lide, 1997).
The ionic mobility of most ions is found within a quite narrow range. Exceptions are hydrogen and hydroxide ions, which are 3 to 5 times more mobile than other ions, with the hydrogen ion being the more mobile of the two. This fact is important for the chemistry in the remediation zone.
2.2 ELECTROOSMOSIS
In addition to electromigration of ions, the electrical potential causes water to flow.
The flow-direction will in most cases be towards the cathode because cations in the electrical double layer around the soil particles exert more momentum to the pore
H+ > OH- > Pb2+ = Cd2+ > Fe3+ > Cr2+ > Al3+ > Ca2+ > Cu2+ = Fe2+ > Zn2+ > Na+ 36.2 20.6 7.36 7.36 7.06 6.94 6.32 6.17 5.6 5.6 5.47 5.19
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fluid than do the fewer mobile anions (Yeung and Datla, 1995). At low pH-values, however, charge reversal of the soil may occur, and cause the electroosmotic flow to change direction. The electroosmotic flux can be described by the following equation (Acar and Alshawabkeh, 1993):
Jje = -ke c c
j w
∇U, (6)
where ke is the electroosmotic permeability and cw the concentration of water (≈ 1).
Because the electroosmotic flow does not depend on the hydraulic conductivity, it may contribute significantly to the fluid flow in soils with low hydraulic conductivity.
The electroosmotic mobility is usually in the order of 5⋅10-9 m2/V⋅s (Lageman et al., 1989). For dissolved ionic elements moving with the water, this is an order of magnitude smaller than the ionic mobility, thus electroosmosis is important mainly for transport of large ions and neutral species. In EDR the electroosmotic transport is reduced significantly by the application of membranes, making EDR suitable for selective transport of small, charged species.
2.3 ELECTROPHORESIS
Electrophoresis refers to the movement of charged particles in water in an applied electric field (Lageman et al., 1989). The electrophoretic mobility varies between 0.1⋅10-9 and 3⋅10-9 m2/(V⋅s) (Lageman et al., 1989). For ionic elements, this is more than one order of magnitude smaller than the electrokinetic mobility, and the process is rarely encountered in EKR/EDR.
2.4 ELECTRODE REACTIONS
As the current passes and various substances are removed from the soil, the chemical equilibrium among the soil-phases is shifted and physico-chemical reactions such as adsorption/desorption and dissolution/precipitation become important. The chemical equilibrium is in particular affected by the dominating electrode reactions:
Application of a DC electric field to inert electrodes immersed in water induces water-splitting reactions and vaporization of gases according to equations (7) and (8).
Anode: 2 H2O O2(g) + 4H+ + 4e- (7)
Cathode: 2H2O + 2e- H2(g)+ 2OH- (8)
At the anode, reaction (7) results in production of hydrogen-ions, while at the cathode (8) hydroxide-ions are produced. In EKR, these ions enter the soil and result in the development of an acidic front evolving from the anode towards the cathode, and an alkaline front evolving from the cathode towards the anode. In EKR intrusion of the alkaline front into the soil is most often hindered by neutralization of the catholyte with acid. In EDR an anion-exchange membrane between soil and anolyte, and a cation-exchange membrane between soil and catholyte hinders the intrusion of the electrolyte-products into the soil as illustrated in figure 2.1. Despite the introduction of these membranes, an acidic front is still developing within the soil specimen due to water-splitting at the surface of the anion-exchange membrane. This process is described in further detail in chapter 6 of this thesis. Depending on the present metals
Application of Microbial Products and the electrode potential, electrodeposition of metals may in addition occur at the cathode:
Cathode: Me2+ + 2e- Me (9)
OH-
Figure 2.1: Principles of Electrodialytic Soil Remediation.
The nature of the described processes, imply that application of microbial products preferentially should result in formation of small, charged Pb-species, which are soluble under neutral/acidic conditions to make electromigrative transport out of the soil as efficient as possible.