Fundamentals of electrokinetics - In-Situ Remediation of Arsenic-Contaminated Sites - page 92

for example, electrokinetic remediation was previously considered as “not applicable” because
transport by electroosmosis is not to be expected (Yang and Lee, 2009). However, the electroki-
netic technology coupled with methods to increase the solubility of HOCs, has been developed
and frequently applied. The PAHs, representative of HOCs, are very hydrophobic and have quite
low aqueous solubility. The solubilization/desorption and fractionizing of PAHs in soil-water
systems have been extensively studied using solubility-enhancing solutions such as surfactants
and cosolvents to achieve effective removal of PAHs from contaminated sites (Yang and Lee,
2009). Electromigration is the dominant transport mechanism for the removal of inorganic con-
taminants, but electroosmosis becomes significant in removing organic contaminants due to their
nonpolar characteristic. Refer to Reddy and Cameselle (2009b) for more detailed information
about electrokinetic removal of organic compounds.
5.2.3.3 Enhancement schemes for electrokinetic soil remediation
In order to mobilize and solubilize contaminants, various enhancement techniques have been
proposed and used (Alshawabkeh et al ., 1999; Kim and Kim, 2002; Page and Page, 2002). For
inorganic contaminants, these include: (i) injection of enhancing agents such as acetic acid or
use of a hydroxyl ion selective membrane in the cathode reservoir to prevent precipitation or to
solubilize precipitates of cationic metal contaminants near the cathode, (ii) conditioning the anode
and/or the cathode reservoirs to control pH and zeta potential, to enhance desorption, to increase
the electroosmotic flow rate, and finally to increase mobility of contaminants, and (iii) adding
or mixing strongly complexing agents, such as ammonia, citrate, and EDTA into soil, which
compete with soil particles for metal contaminants to form soluble complexes. Among these
enhancement technologies, the scheme for prevention of metal precipitation has been mostly
focused and experimentally evaluated. Alshawabkeh (1994) summarized the characteristics of
enhancement schemes as follows: (i) the precipitate should be solubilized and/or precipitation
should be avoided; (ii) ionic conductivity across the soil specimen should not increase excessively
in a short period of time both to avoid a premature decrease in electroosmotic transport and to allow
transference of species of interest; (iii) the cathode reaction should possibly be depolarized to
avoid generation of the hydroxide and its transport into the soil specimen; (iv) such depolarization
will also assist in decreasing the electrical potential difference across the electrodes leading to
lower energy consumption; (v) if any chemical is used, the precipitate of the metal with this new
chemical should be perfectly soluble within the pH ranges attained, and (vi) any special chemicals
introduced should not result in any increase in toxic residues in the soil mass. Recently, Yeung and
Gu (2011) gave a comprehensive review on the techniques to enhance electrokinetic remediation
of contaminated fine-grained soils. They summarized enhancement agents developed so far, such
as chelants, complexing agents, surfactants and cosolvents, oxidizing/reducting agents, and cation
solutions, and also explained the methods for electrode conditioning and use of an ion exchange
membrane to control soil pH during the process.
5.2.3.4 Implementation of electrokinetic remediation
A typical system for electrokinetic remediation is presented in Figure 5.2 . The system mainly
consists of two unit processes: electrokinetic removal and effluent treatment processes. The imple-
mentation of electrokinetic removal of metal contaminants is exemplified here. Most of the metal
contaminants are positive ionic compounds in a soil-water-electrolyte system, and they migrate
towards the cathode when an electric field is applied to the system. Furthermore, these metal con-
taminants are removed through the cathode effluent solutions from the contaminated soils. The
cathode effluent solutions should be properly treated in order to recover metals contained in these
solutions. The typical ranges of electric field strength and current density for electrokinetic reme-
diation are known to be 1-100 V m − 1 and 1-10 A m − 2 , respectively (Alshawabkeh et al ., 1999).
Although implementation of the electrokinetic remediation system is relatively simple, its design
and operation for successful remediation is cumbersome due to complex dynamic electrochemical
transport, transfer, and transformation processes that occur under applied electric potential. In