Electrodeionization

To maximize the purity of the product, EDI feedwater needs pre-treatment, usually reverse osmosis. When fed with low total dissolved solids feedwater (e.g., purified by RO), the product can reach very high purity levels, with conductivity on the order of 0.5 S/cm. Feedwater must follow certain requirements to prevent damage to the instrument.

Common parameters are:

• Hardness of feedwater: 1 ppm as CaCO_3, with limited exceptions up to 2 ppm.
• Silica content (SiO₂) must be 1 ppm in most EDI cells or 2 ppm in thin-cell modules.
• CO₂ must be monitored to prevent excessive loading of anion exchange resin.
• TOC, which can foul resins and membranes, must be minimized.
• Chlorine, ozone, and other oxidizers can oxidize resins and membranes and create permanent damage, which must be minimized.

To eliminate or minimize the concentration polarization phenomenon present in electrolysis systems of the time, electrodeionization was developed in the early 1950s. A patent on the technology was filed in 1953, and subsequent publications popularized the technology.[2]

The technology was limited in application because of the low tolerance of total dissolved solids, hardness and organics. During the 1970s and 1980s, reverse osmosis became a preferred technology to ion exchange resin for high TDS waters. As RO gained popularity, it was determined that EDI would be a suitable polishing technology. Packaged RO and EDI systems were used to displace chemically regenerated ion exchange systems.

In 1986 and 1989, several companies developed electrodeionization devices. The initial devices were large, costly, and often unreliable. However, in the 1990s, smaller and less costly modular designs were introduced, some of which minimized leakage. Nonetheless, these designs and their contemporary descendants still face limitations such as cost and limited operational envelope.[3][4]

In electronics, deionized water is used to rinse components during manufacturing. The electronic chips are very small, with little free space between component elements. Therefore, very low numbers of ions are necessary to conduct unwanted electricity between components. If this occurs, a short circuit may result and make the chip unusable.

In the pharmaceutical industry, the presence of ions in the water used in drug development can lead to unwanted side reactions and introduce harmful impurities.

In power generation, the presence of ions in boiler feedwater can lead to the buildup of solids or the degradation of boiler walls, both of which can lower boiler efficiency and present safety hazards. Due to the large financial and safety concerns present in these industries, they provide the bulk of the revenue for EDI developers.

Electrodeionization systems have also been applied to the removal of heavy metals from different types of wastewater from mining, electroplating, and nuclear processes. The primary ions removed in these processes are chromium, copper, cobalt, and caesium, though EDI sees use in the removal of others as well. [5]

Electrodes in an electrochemical cell are referred to as either anodes or cathodes. The anode is defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode is 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.

Each deionization cell consists of an electrode and an electrolyte with ions that undergo either oxidation or reduction. Because they commonly consist of ions in solution, the electrolytes are often known as "ionic solutions", but molten and solid electrolytes are also possible.

Water is passed between an anode and a cathode. Ion-selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. As a result, the ions cannot escape the cell, and deionized water is produced. [4]

When using a current that is higher than what is necessary for the movement of the ions, a portion of the incident water will be split, forming hydrogen (OH-) and oxygen (H+) ions. This species will replace the impurity anions and cations in the resin. This process is called "in situ regeneration" of the resin. As it occurs during the deionization process, it has the benefit of allowing for continuous purification as opposed to requiring a pause in operation to manually chemically regenerate ion exchange resin in other deionization techniques. [6]

Electrodeionization installation scheme

The typical EDI installation has the following components: electrodes, anion exchange membranes, cation exchange membranes, and resin. The most simplified configuration comprises three compartments. To increase production, the number of compartments or cells can be increased as desired.

Once the system is installed and feedwater begins to flow through it, cations flow toward the cathode and anions flow 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 flow in only one direction because of the selectivity of the membranes and the electric force, leaving the feedwater free of ions. It also allows for the separate collection of cation and anion concentration flows, creating the opportunity for more selective waste disposal; this is especially useful in the removal of heavy metal cations.

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

The purpose of the ion exchange resin is to maintain the stable conductance of the feedwater. Without the resin, ions can be removed initially, but the conductance will drop dramatically as the concentration of ions decreases. With lower conductance, the electrodes are less able to direct the flow of electrons and remove them; with the addition of resins, a steady rate of removal is possible, and the remaining ion concentration in the processed water is lower by orders of magnitude. [5]

• video.
• Bộ lọc nước phèn
• Electrodeionization Technology
• EDI History
• Electrodeionization Systems, Electrodeionization Systems
• Advanced Electrodeionization Technology for Product Desalting, Argonne National Laboratory