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Electrokinetic Remediation - Physical and Chemical processes involved

 

Physical and Chemical processes involved

The physical and chemical processes involved are electrolysis, diffusion, adsorption-desorption, precipitation-dissolution and oxidation-reduction.

 

Electrolysis:

This is the reaction that occurs at the anode and cathode when an electric field is applied. In the case of electrokinetic remediation, water decomposition occurs at the electrodes. Oxidation takes place at the anode and hydrogen ions H+ and oxygen gas are generated. Reduction reaction takes place near cathode and hydroxyl ions OH- and hydrogen gas are generated. These are shown by the following equations (Cameselle et al., 2013; Sharma and Reddy., 2004).

6.equ

 

The process of electrolysis leads to changes in the soil pH near the electrodes. The region near anode develops a low pH of about 2 wheareas near the cathode it increases up to 11 or 12. The hydrogen and hydroxyl ions move both due to electromigration and diffusion. The hydrogen ions being smaller than the hydroxyl ions tend to travel faster leading to rapid acid front migration at almost twice the speed of the base front migration. The acid front migration is even easier in low buffering capacity soils compared to high buffering capacity soils since the hydrogen ions get consumed to neutralize soil constituents (Sharma and Reddy., 2004). The acid dissolves the usual cations in the soil or precipitates and helps cation removal. If the contaminants are anionic, the acid front would increase adsorption and reduce contaminant removal. Both the acid and base front will in turn have an effect on the zeta potential of the soil influencing the flow.

 
Diffusion

Diffusion is the process where the contaminants move due to difference in concentration gradient. This usually does not contribute to a significant amount of contaminant transport in remediation. This depends on the tortuosity and porosity of the medium and the concentration of the species (Sharma and Reddy., 2004). The rate of diffusion is given by:

 4.ele

 
Adsorption-Desorption:

Adsorption involves the movement of contaminants from pore water to the soil particles. Generally soils are negatively charged but it would depend on the pH of the soil (Reddy., 2011). The net charge in soil will be zero for a particular value of pH and that is called the point of zero charge (PZC). If the pore water falls to a pH less than PZC then adsorption of anions will be predominant and when pH rises above PZC the adsorption of cations are significant. Thus adsorption is dependent on factors including soil type, soil charge, contaminant, organic matter and pore water characteristics.

Desorption is the reverse of adsorption and involves the transport of contaminants from soil to pore water. When pH falls below PZC desorption of cations are significant and vice versa. Due to extreme pH differences between the regions near the electrodes, the cationic absorption and anionic desorption occurs near cathode. The anionic adsorption and cationic desorption occurs near anode. The area of the PZC is known as the slippage plane and beyond this, free porewater is present in the soil. 

 
Precipitation-dissolution:

Precipitation is the formation of solid that results when the concentration of compound exceeds its solubility. Dissolution is the reverse where compound forms a solution. Both processes are highly pH dependent and would occur depending on their location. The contaminants could be precipitated or dissolved during remediation. The dissolved contaminants would be easier to remove than the precipitated contaminants in soil.

 
Oxidation-Reduction:

Redox reactions take place during the remediation process. Area near Anode experiences oxidation since electrons are lost and the cathode area experiences reduction since there is addition of electrons. Some metal cations precipitate near cathode. The valence of the metal ions decides on their solubility and thus might impact removal (Sharma and Reddy., 2004).

 

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