Electrodeionization

Electrodeionization (EDI) is usually considered a water treatment technology that utilizes an electrode to ionize water atoms and separate dissolved ions (impurities) from water. It differs from other water purification technologies in that it is done without the use of chemicals and is usually a tertiary treatment to reverse osmosis(RO).

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How It Works

An electrode in an electrochemical cell (battery), is referred to as either an anode or a cathode, words that were coined by Michael Faraday. The anode is defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode as the electrode at which electrons enter the cell and reduction (Redox) occurs. Each electrode may become either the anode or the cathode depending on the voltage applied to the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell.

Each cell consists of an electrode with atoms, and an electrolyte with ions that undergo either oxidation or reduction. An electrolyte substance containing free ions that behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. They are sometimes referred to in abbreviated jargon as lytes.

Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity deionized water results.

Under Direct Current (DC) electrical current, Water (H2O) behaves as follows:

H₂O -> H+ + OH-

Effects on pH

This can have a marked effect on pH of the solution as well. pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25°C with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). When a pH level is 7.0, it is defined as 'neutral' at 25°C because at this pH the concentration of H₃O+ equals the concentration of OH− in pure water. pH is formally dependent upon the activity of hydronium ions (H₃O+),(H+ is often used as a synonym for H₃O+.) Because pH is dependent on ionic activity, a property which cannot be measured easily or fully predicted theoretically, it is difficult to determine an accurate value for the pH of a solution.

References

• CEDI University web site
• Electrodeionization (EDI), The Dow Chemical Company

See also

• Ionization