Principles of the technology

Electrokinetics has emerged as one of the most versatile technologies for soil remediation over the past three decades because of its suitability to treat both inorganic and organic pollutants as well as radionuclides, saturated and unsaturated soil matrices, low permeable soil and heterogeneous soil layers.

Electrokinetic treatment has shown its potential also for the simultaneous removal of inorganic and organic species from soil [3, 4]. Electrokinetics is a process that can be used to decontaminate soil by driving the inorganic and organic contaminant species through the soil matrix. The driving force for this species transfer is the applied electric field. Finally, the contaminants are removed by electroplating at the electrode, precipitation or co-precipitation at the electrode, adsorption onto the electrode or complexing with ion exchange resins (heavy metal species) and pumping water near the electrode (organic contaminants) [5].

Several large scale applications of electrokinetics were reported even from late 1970s and early 1980s [6]. These studies were based on the fundamental aspects of the technology without the comprehension of complicated electrochemical phenomena that actually govern the process.

However, the results of these applications proved the potential of electrokinetics for the removal of a wide range of inorganic pollutants, soluble organic pollutants and radionuclides [7, 8, 9, 10, 11, 12]. Later on, the focus was directed to the enhancement of the processes for better removal rates, including pH conditioning and surfactant additions [13, 14, 15]. At present, several pilot

scale trials are being carried out for the removal of organic compounds, heavy metals and radio nuclides from contaminated soil. [16, 17, 18]

The recent developments and the history of electrokinetics, how it emerged and evolved as a remediation technique, have been explained by Yeung [19].

The principles of electrokinetic technology have elaborately been explained and thoroughly understood [2, 5, 20, 21]. There are several electrochemical phenomena taking place upon the application of electric field on a soil mass. However, the major transport mechanisms that are relevant from a remediation standpoint are the following:

1. Electromigration 2. Electroosmosis 3. Electrophoresis

Electromigration refers to the movement of charged species present in the soil mass under the influence of an externally applied electric field. The charged species move towards the electrodes of opposite polarity. The ionic mobilities of heavy metals at infinite dilution are in the range of 10 ^-4 cm²/Vs. However, taking into account the effective ionic mobility due to the tortuosity in a porous medium like soil, the rate of heavy metal transport in clayey soil is about a few centimeters per day under a unit electric gradient [22].

Electroosmosis is the movement of the pore fluid under an electric field which results from the interaction between the bulk liquid and the diffuse double layer existing at the soil particle /fluid interface. The direction of the electroosmosis depends on the surface charge of the soil particles.

Since, a negative surface exists in most soil particles, especially on clayey soil, excess positive charges are distributed adjacent to the soil surface which continuously drags the bulk fluid

towards the cathode. Therefore, the direction of electroosmosis is always towards the electrode of negative polarity unless, the surface charge of the soil particles are changed. The rate of electroosmotic flow in a porous medium is defined by the Helmholtz-Smoluchowski equation which is explained in latter section.

Electrophoresis is the transport of charged particles (like clay particles or microorganisms) or colloids under an applied electric field. However, electrophoresis has no major role in a compact solid phase, where there is minimal movement of the particles.

A recent review by Mahmoud et al. [23] presents a detailed account of these electrokinetic transportation phenomena.

Highly soluble ionized inorganic species that are present in moist soil environments are transported by electromigration and also by electroosmosis depending upon the species concentration [5]. However, electroosmosis plays the dominant role in the transport of soluble organic species present in the soil.

Besides these transportation processes, the application of voltage also leads to other electrode reactions and the corresponding geochemical reactions in the treated material. The electrode reactions involve the electrolysis of water:

2H2O – 4e^- = O2 + 4H⁺ (anode) (1)

2H2O+ 2e- = H2 + 2OH^- (cathode) (2)

The protons and hydroxyl ions generated at the anode and cathode, respectively, are transported through the soil. The mobility of protons under an applied electric field is about two times the mobility of hydroxyl ion [24]. Also, the advance of the basic front developed at the cathode is

retarded by the counteracting electroosmotic flow. Therefore, the soil pH that develops during an electrokinetic process depends on the extent of the movement of protons and hydroxyl radicals as well as the geochemical characteristics of the soil such as its buffering capacity [25]. These changes in the soil pH lead to other geochemical reactions in the soil which include sorption-desorption reactions, complexation reactions, precipitation –dissolution reactions and oxidation-reduction reactions.

These geochemical reactions significantly affect the electrokinetic process and can enhance or retard the process [22].

Several techniques have been proposed to alter the normal geochemical reactions that arise from the electrode reactions to enhance the electrokinetic removal of species from the soil. They are controlling the soil pH by suitable acid, base, and buffer additions into the anolyte or catholyte, by using specially designed ion selective membranes, or by using enhancement agents which aid in the complexation and removal of species present etc [15, 22 ].