Electrodeionization

Electrodeionization is a water treatment technology that utilizes electricity, ion exchange membranes and resin to deionize water and separate dissolved ions (impurities) from water. It differs from other water purification technologies in that it is done without the use of chemical treatments and is usually a polishing treatment to reverse osmosis (RO). There are also EDI units that are often referred to as continuous electrodeionization (CEDI) since the electric current regenerates the resin mass continuously. CEDI technique can achieve very high purity, with conductivity below 0.1 µS/cm. Recently, Argonne National Laboratory developed a process called Resin-Wafer Electrodeionization (RW-EDI), which uses a unique porous resin wafer mold made from immobilized loose ion-exchange resin beads. The resin wafer material enhances mass transfer between solid (resin bead) and liquid (feed solution) phases to achieve a high purity, especially when treating impaired or[1] brackish water.

Applications

When fed with low TDS feed (e.g., feed purified by RO), the product can reach very high purity levels (e.g., 18 Megohms/cm[2]). The ion exchange resins act to retain the ions, allowing these to be transported across the ion exchange membranes. The main applications of EDI technology, such as that supplied by Ionpure, E-cell and SnowPure, are in electronics, pharmaceuticals and power generation.

One important aspect in the water treatment application is that some types of EDI needs to have feed water that is free from CO2, as well as other dissolved gasses, since these put unnecessary strain on the EDI unit and will reduce performance.

Theory

An electrode in an electrochemical cell is referred to as either an anode or a cathode, terms 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 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 and an electrolyte with ions that undergo either oxidation or reduction. An electrolyte is a 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 an anode (positive electrode) and a cathode (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.

Resin-Water Electrodeionization

RW-EDI is a process that targets the desalination of impaired water or water with salt levels of 1,000 - 10,000 ppm. RW-EDI process uses a porous ion exchange resin wafer with 195 cm2 cross-section surface area. Water is fed through the wafer, while an electric current is applied to setup. In between resin wafer compartments, there are concentrate compartments, where brine flows out of the system. An anode is setup on the left side of the setup and a cathode is setup on the right side of the setup. The electric current (supplied from various energy sources) charges the ions that make up the contaminants. The positively charged ions flow toward the cathode and are rinsed out in the concentrate stream, and the negatively charged ions flow toward the anode and are rinsed out in another concentrate stream. Purified water flows out through the opposite side of the compartment. The resin-wafer technology increases the energy efficiency of the desalination process significantly, especially when testing impaired water. RO processes have an energy efficiency of 4%, whereas RW-EDI has an energy efficiency ranging from 35-65%. Argonne National Laboratory estimates that 402.83 GWh of energy can be saved a day, if this technology is implemented for the processing of wastewater from industrial plants.[3]

See also

References

  1. ↑ Pan, Shu-Yuan; Snyder, Seth W; Ma, Hwong-Wen; Lin, Yupo J; Chiang, Pen-Chi (2017). "Development of a Resin Wafer Electrodeionization Process for Impaired Water Desalination with High Energy Efficiency and Productivity". ACS Sustainable Chemistry & Engineering. 5 (4): 2942–2948. doi:10.1021/acssuschemeng.6b02455.
  2. ↑ , Resistivity / Conductivity Measurement of Purified Water, Lab Manager Magazine
  3. ↑ Pan, Shu-Yuan; Snyder, Seth W; Ma, Hwong-Wen; Lin, Yupo J; Chiang, Pen-Chi (2017). "Development of a Resin Wafer Electrodeionization Process for Impaired Water Desalination with High Energy Efficiency and Productivity". ACS Sustainable Chemistry & Engineering. 5 (4): 2942–2948. doi:10.1021/acssuschemeng.6b02455.
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