Removal of Anionic Pollutants by Electrokinetics

Fertilizer concentration is also a problem in greenhouses. A high concentration of salts in the soil inhibits water movement from soil to plant due to high osmotic pressure between pore water and plant root. 

At low-level concentrations, fluoride can reduce the risk of dental cavities. Exposure to somewhat higher amounts of fluoride can cause dental fluorosis, which, in its mildest form, can result in the discoloration of teeth. Severe dental fluorosis produces pitting and alteration of tooth enamel. Higher intake of fluoride, taken over a long period of time, can result in changes to bone and in a condition known as skeletal fluorosis. The effects of skeletal fluorosis include joint pain, restriction of mobility, and a possible increase in the risk of some bone fractures. 

In electrokinetic processes, there are two major transport mechanisms electromigration and electro-osmosis. Generally, in an electrical field, electromigration causes cationic metals such as cadmium, zinc, lead, nickel, and copper to move from the anode toward the cathode in electro-osmosis, the direction of movement of the pore water is toward the cathode when the zeta potential of the soil surface is negative. This can result in an enhanced removal of metals because the direction of transport of the ions in both mechanisms is the same. However, the direction of electromigration for anionic pollutants is toward the anode and that for electroosmosis is from anode to cathode, as stated previously. The opposite direction of movement means that the removal rate of anionic pollutants could be reduced. 

Soil pH plays an important role in the adsorption and desorption of pollutants and, ultimately, influences transport. Generally, the desorption of cationic metals would be enhanced by acidic conditions because of ion exchange reactions between hydrogen ions and cationics. However, anionic pollutants would be desorbed more easily in an alkaline condition than in an acidic environment, indicating that the process fluid should be changed for the electrokinetic removal of anionic pollutants. 

Electrokinetics has been applied to remove nitrate from soil to protect groundwater and to enhance bioremediation of soil contaminated by other pollutants. In this section, nitrate is treated as a pollutant in Section 6.3.3, it is treated as a nutrient. 

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