Electrodeionization (EDI)

Electrolysis, electrodialysis, and electrodeionization 

Electrolysis involves the passage of an electrical current through an electrolyte solution, with the subsequent movement of positively and negatively charged to negative and positive electrodes. This process effectively splits water molecules, and is the driving force of electrodeionization. Electrodialysis then separates the hydroxal (OH-) and hydrogen (H+) ions from the electrolyte solution, while electrodeionization overcomes the limits of electrodialysis, allowing for ion separation without increasingly higher voltage.

The fundamental process is this: Imagine a simple model of a battery connected to two electrodes immersed in a saltwater bath. When charge is introduced on the electrodes, a reduction reaction involving the water molecules occurs at the cathode: hydrogen gas is released and OH- ions are left behind. At the anode, oxygen gas is released and H+ ions are left behind in an oxidation reaction. The presence of salt in the solution facilitates continuous reactions at the electrodes, drawing hydroxyl ions from the cathode and hydrogen ions from the anode. 

In electrodialysis, electrical current drives ions across a semipermeable membrane. In an EDI system, a membrane that allows for the passage of cations (OH- ions) only is positioned next to the cathode, and a membrane permeate to anions (H+ ions) only is positioned next to the anode. A central chamber now contains the saline solution. When the electrical charge is applied to the system and the chemical reactions occur, the ions will move across the membranes out of the central chamber to their respective electrodes, leaving behind the constituents of the salt molecules and any other impurities.

Electrodialysis is limited, however. As the water becomes purer, the system’s voltage requirement increases — even exceeding 600 volts — which can cause arching. Electrodeionization resolves this challenge by introducing ion exchange (IX) resins, or ionically conductive media, into the central chamber. This allows the ions to easily migrate out of the central, dilute chamber without high voltage.

Using DuPont™ EDI modules

When installed as part of a water-treatment system, electrodeionization modules from DuPont Water Solutions use electrical current to force a continuous migration of contaminant ions out of the feedwater and into the reject or concentrate stream, while continuously regenerating the ion exchange (IX) resin bed with H+ and OH- ions that are derived from water splitting.

Feedwater (the dilute stream) enters from the bottom of the DuPont™ EDI module and is diverted into vertically spiraled cells known as dilute chambers. The dilute streams flow vertically through ion exchange resins located between the anion and cation membranes. 

Concentrate enters the bottom of the module through the center pipe and is diverted into spirally flowing cells known as concentrate chambers. DC current is applied across the cells, splitting a small percentage of water molecules into H+ and OH- ions. Drawn to their respective electrodes, the H+ and OH- ions first migrate through their respective resins, continuously regenerating the resin, then through their respective permeable membranes and into the concentrate chambers. Contaminate ions dissolved in the feedwater then attach to their respective ion exchange resins, displacing H+ and OH- ions. Once within the resin bed, the ions join in the migration of other ions and permeate the membrane into the concentrate chambers. The contaminant ions are trapped in the contaminant chambers and are recirculated and bled out of the system. 

The feedwater continues to pass through the dilute chamber, is purified and collected on the outlet of the dilute chambers, and then exits the DuPont™ EDI module.  

Conversion of carbonic and silicic acids

Electrodeionization is an effective means of polishing reverse osmosis (RO) permeate. When the permeate water enter the EDI system with the electric current applied, some of the permeate flows through the dilute chamber, where most of the cation and anions are removed. As the water become purer, the voltage differential begins to exceed 2 volts, splitting the water molecules. This creates localized areas of low pH (with H+ ions) and high pH (with OH- ions). In the areas of high pH, carbonic acid can convert to the anion bicarbonate and silicic acid to the anion bisilicate, which EDI can remove.