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Electrokinetic Remediation - Types of Contaminant Transport Mechanisms

 

 

Types of Contaminant Transport Mechanisms

Electrokinetic Remediation will lead to the transport of the contaminants through four main mechanisms. These four types of transport mechanisms are affected by physical-chemical processes as described below:

Electroosmosis:

Electroosmosis occurs due to Columbic forces which are induced by the applied electric field in the soil.  The movement of contaminants is by advection in the moist soil. The soil particles are usually negatively charged and the positively charged cations in pore water align along the negative soil surfaces (Virkutyt., 2002). The water molecules in turn align around the cations till there are no excess cations left. The remaining water molecules end up around the negatively charged soil surface forming the boundary layer. The closest molecules are held by electrical attraction though they are free to move in the double layer (Sharma and Reddy., 2004).

 

When we apply electric field during remediation, the positively charged water molecules will move towards the negatively charged cathode. As the cations move toward the negatively charged electrode, a shearing action takes place. The cations move through the pore water at high velocities and pull the water molecules along with them. This results in the large transfer of water from anode to cathode (Jane E. Apatoczky, 1992). The water molecules movement in the double layer is affected by Zeta potential which is actually the potential that develops between moving and stationary parts of Double layer around soil particles. The potential usually is negative and ranges between -10 to -100mV for saturated silts and clays. The flow would be reversed if the zeta potential is positive and this can happen if the contaminant concentration is high enough. In electroosmosis, flow depends on temperature, ion concentration, viscosity of pore water, dielectric constant and ion mobility. It is denoted as:

2

 

Electromigration

This is the motion of ions towards the electrodes as shown in figure 2. This transport mechanism is comparatively faster than the advective transport of electroosmosis and is the dominant transport mechanism during remediation. The rate of movement of ions is a function of ionic mobility, valence numbers and electrolyte concentration (Virkutyt., 2002; Sharma and Reddy., 2004). The ions travel with a velocity given by:

3. elecmi

 

Electrophoresis

This is the movement of colloids towards electrodes and is similar to electromigration (Virkutyt., 2002). This type of movement is insignificant if the soil is tightly packed which would physically restrict the colloids from moving.

 

 

5.ion trnas

Figure 2: Transportation of ions in the soil (Cameselle et al., 2013)

 

The other electrokinetic mechanisms that occur during remediation include transportation due to streaming potential. The streaming potential develops when electrolyte is driven by pressure gradient through pores with charged walls.

 

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