Electrodeionization

Electrodeionization (EDI) 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-Water Electrode ionization (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.

History

In order to eliminate or minimize the concentration polarization phenomenon present in electrodialysis systems, electrodeionization originated in the late 1950s. In 1955, researcher Walters et al, at the Argonne National Laboratory, developed one of the first Descriptions of electrodeionization using this method to remove traces of radioactive elements in the water.

After 10 years, Sammons and Watts (Harwell Atomic Energy Authority) explored a way to use an EDI module to deionize a saline solution (NaCl). Nervertheless, several factors were not considered for cell design, therefore, an EDI device could not be adequately developed.

The evolution of deionization technology progressed slowly in the early years, and in 1971 Matejka applied this system to obtain high purity water. In 1986 and 1989, companies like Millipore Co. and Ionics Inc. developed electrodeionization devices. Later, the researchers focused on investigating how other systems could be combined with the EDI system and what types of membranes would allow better results, with which the field of application of electrodeionization grew and focused not only on obtaining high purity water .

Presently, this technology allows to segregate almost completely the ionic species in the diluted solutions and the toxic metallic ionic species that are present in the effluents of industrial waste, for this reason it is applied in the treatment of residual waters in the pharmaceutical, semiconductor and energy sectors. [2]

Applications

When fed with low total dissolved solids (TDS) feed (e.g., feed purified by RO), the product can reach very high purity levels (e.g., 18 megohms/cm).[3] 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 CO
2, 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

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.[4]

In situ regeneration

When using an excess of current that is higher that the necessary for the movement of the ions. A portion of the water will be splitted forming OH- and H+. This species will replace the anions and cathodes in the resin, this process is called regeneration in situ of the resin. And because it occurs during the process itself there is no need to stop the installation and use chemicals as it happens in others techniques.[2]

Installation scheme
Electrodeionization installation scheme

The typical RW-EDI installation has the following components: anode and cathode, anion exchange membrane, cation exchange membrane and the resin. The most simplified configuration consist in 3 compartments, to increase the production these number can be increased.

The cations flow towards the cathode and the anions flows toward the anode. Only anions can go through the anion exchange membrane and only cations can go through the cation exchange membrane. This configuration allows anions and cations to only flow in one direction because of the membranes and the electric force, leaving the feed water free of ions, (deionized water).

The concentration flows (right and left of the feed flow) are rejected and they can be wasted, recycled, or use in another process.

The purpose of the ion exchange resin is to maintain stable conductance of the feed water. Without the resins, the conductance will drop dramatically as the concentration of ions is decreasing. Such drop off of conductance makes it very difficult to eliminate the 100% of ions. But using resins makes it possible.

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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. Alvarado, Lucía; Chen, Aicheng (2014-06-20). "Electrodeionization: Principles, Strategies and Applications". Electrochimica Acta. 132: 583–597. doi:10.1016/j.electacta.2014.03.165. ISSN 0013-4686.
  3. , Resistivity / Conductivity Measurement of Purified Water, Lab Manager Magazine
  4. 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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