Fundamentals of electrokinetics - In-Situ Remediation of Arsenic-Contaminated Sites - page 95

distribution (relative proportion of particle size), porosity, tortuosity, water content (or degree of
saturation), activity, heterogeneity, and electrical properties, such as zeta potential of the surface,
electroosmotic permeability and electric conductivity. In terms of chemical properties, adsorption
capacity (or ion exchange capacity), pH, pH buffering capacity, and organic content should be
evaluated prior to application of electrokinetic remediation. In addition, the mineralogy of the
soil definitely affects the process. Particularly, the type and content of oxides, carbonates, and
clay minerals contained in the soil is crucial because it affects numerous properties of soil such as
adsorption capacity, electroosmotic permeability, and pH buffering capacity. All those properties
of soils control electromigration of contaminants as well as electroosmosis, and finally affect
the overall performance of the electrokinetic process. For example, soils of high water content,
high degree of saturation, and low activity provide the most favorable conditions for transport of
contaminants by electroosmotic advection and electromigration. However, soils of high activity
(e.g., montmorillonite) exhibit high pH buffering capacity, and require excessive acid and/or
enhancement agents to desorb and solubilize contaminants sorbed on the soil particle surface
before they can be transported through the surface and removed (Alshawabkeh et al ., 1999).
Consequently, the properties of soil are most crucial in electrokinetic remediation and should be
evaluated prior to implementation.

5.3.1.2 Characteristics of contaminants
The characteristics of contaminants affecting electrokinetic remediation are type, concentration,
mobility, and chemical form. The physico-chemical interaction between soil media and contam-
inants depends on the types of contaminants such as inorganics (heavy metals, metalloid, and
radionuclides) and organics (PAHs, pesticides, herbicides, etc.). Type of contaminant also deter-
mines whether the main mechanism for transport is electromigration, electroosmosis, or coupled
effect of both. In the case of heavy metal contaminants, for example, electromigration and elec-
troosmosis play a simultaneous role in transporting and removing the contaminants under normal
conditions of soils. However, non-polar organic contaminants are best removed by electroosmotic
flushing rather than electromigrative transport, without considering the oxidation effect induced
by the electric field. The concentration of contaminants influences the efficacy of the process
as well. Removal and current efficiencies can be decreased if the concentrations of non-target
contaminants are higher than those of target contaminants. In order to explain this phenomenon,
the transport (transference) number is introduced. The transport number ( t i ) of the contaminant i
is given as (Alshawabkeh, 1994; Kim, 2001):
t i = ( z i u i C i ) / ( z i u i C i ) (5.13)

where z i , u i , and C i are the charge, the effective ionic mobility, and the aqueous concentration of
the contaminant i , respectively. The transport number gives the contribution of the i -th on to the
total effective conductivity. The summation of transport numbers of all ions in the soil pore fluid
should be equal to one. Equation (5.13) formalizes the dependence of the transport number of an
individual ion on its effective ionic mobility, concentration, and total electrolyte concentration in
the pore fluid. The transport number of a species will increase as the ionic concentration of that
specific species increases. It indicates that as the concentration of a species decreases relative to
the total electrolyte concentration in the pore fluid, its transport and removal under an electric
field will be less efficient. Therefore, it is reasonable to assume that the efficiency of removal
of a specific contaminant will decrease in time as its concentration in the pore fluid decreases
(Alshawabkeh, 1994). In addition to the concentration of contaminants, their chemical form (or
speciation) affects the efficiency of the process because it is directly related to the mobility and
solubility of contaminants. Depending on the conditions of the surrounding environment, the
contaminants are partitioned into various forms, such as (1) a dissolved fraction in pore fluid, (2)
a water soluble or exchangeable fraction, (3) a specifically adsorbed fraction, (4) a precipitated
fraction as insoluble carbonates, sulfides, phosphates, and oxides, (5) an organically complexed
fraction, (6) a crystalline (hydro)oxides fraction, and (7) a residual fraction (Kim et al ., 2009a).