Fundamentals of electrokinetics - In-Situ Remediation of Arsenic-Contaminated Sites - page 91

acid and the base front through soil promotes the dissolution of metal ions near the anode and the
precipitation of the metal ions near the cathode (Alshawabkeh, 1994; Kim et al ., 2002a). These
conditions significantly affect the pH and ionic strength of pore water, the mobility and solubility
of metal contaminants, and charge conditions of soil particles. Comparing Equations (5.11) and
(5.12) , the concentration of H + ions is two times higher than that of OH − ions because the electrol-
ysis of 1 mole of water molecule renders 2 moles of H + ions and 1 mole of OH − ions. In addition,
the ionic mobility of the hydrogen ions is twice as high as that of the hydroxide ion, as addressed
in Section 5.2.1.1. Since normal electroosmosis (i.e., from anode toward cathode) occurs in com-
mon pH conditions of soils, the electromigration of hydrogen ions is enhanced by electroosmosis
toward the cathode. For these reasons, the acid front is dominant in soils and decreases soil pH.
A decrease in soil pH is favorable in the electrokinetic removal of cationic contaminants, such
as toxic heavy metals, because of effective desorption and dissolution of contaminants that are
adsorbed on the soil surface and precipitated within soil. Having greater replacing power on the
soil surface than any other species, hydrogen ions in the acid front displace metal ions and organics
adsorbed on the soil surface, resulting in more mobile forms of contaminants in pore water that
can be transported by electromigration. The variation of pH conditions in soils by electrolysis of
water in the electrode compartment has effects on ionic strength of pore water and soil surface
properties such as cation or anion exchange capacity, magnitude and sign of the electrokinetic
zeta potential. Furthermore, speciation, mobility and solubility of contaminants are often varied
with pH in soils during treatment, which may limit or enhance treatment efficiencies (Kim et al .,
2002a).
5.2.3 Fundamental principle of electrokinetic remediation
5.2.3.1 Transport and removal of inorganic contaminants
The electrokinetic technology can be applied to remove inorganic contaminants from soils.
Depending on the chemical and geochemical characteristics, inorganic contaminants are cate-
gorized into three groups: (i) cationic toxic heavy metals such as Cd, Cu, Hg, Ni, Pb, and Zn, (ii)
anionic metals, metalloids, and other inorganics such as As, Cr, Se, NO 3 , and F, (iii) radionuclides
such as Sr and U (Kim and Kim, 2002; Reddy and Cameselle, 2009a). Their behaviors in soils
widely vary depending on the type of contaminants and soil properties. Particularly, the speciation
and transport of the inorganic contaminants are totally controlled by the dynamic changes in the
pH and redox potential of the soil media under applied electric field during the electrokinetic
processing (Virkutyte et al ., 2002). For example, as the acid front migrates through the soil bed,
the cationic contaminants adsorbed on the soil surface are desorbed (Page and Page, 2002). The
free chemical species present in the pore fluid transport towards the electrodes depending on their
charge. Even though there has been some argument about the primary mechanism for removal of
ionic contaminants, electromigration is considered as the dominant driving mechanism contribut-
ing to the transport of inorganic contaminants through the soil mass. However, both precipitation
and sorption retard movement of cationic contaminants at high pH zones. Soil pH changes induced
by the electric field complicate the geochemistry of soil and inorganic pollutant removal. Cations
are collected at the cathode and anions at the anode as a result of the transport of chemical species
in the soil pore fluid. In particular, metal and other cationic contaminants are removed from the
soil with the cathode effluent solution and/or are deposited at the cathode. Hence, the effluent
should be treated for the complete removal and the recovery of the metal contaminants.
5.2.3.2 Transport and removal of organic contaminants
Similar to inorganic contaminants, volatile and soluble organic contaminants can be effectively
removed by electrokinetic technology. Recently, efforts have been focused on the electrokinetic
removal of hydrophobic and persistent toxic organic compounds, such as polycyclic aromatic
hydrocarbons (PAHs), polychlorinated organic compounds, pesticides, herbicides, and energetic
compounds in soils (Reddy and Cameselle, 2009a). In the case of hydrophobic organic com-
pounds (HOCs) with low solubility in water and with a high tendency to be adsorbed onto soil,