<b>What is Electrodeionization?</b><br/><br/>

What is Electrodeionization?

Jamie Grossi

As analytical processes evolve and the pursuit of lower detection limits in the determination of trace contaminants develops, the call for water purification processes for the small scale production of highly purified and cost effective water in the laboratory has been necessitated. Over the past 20 years, one of these emerging water purification technologies has been electrodeionization (EDI).

Figure 1. Picture of a typical cell pair.

EDI, also referred to as continuous deionization (CDI), is a chemical-free process that removes ionized and ionizable species from solution using ion exchange resins that are continuously regenerated by an electric current. Purified water produced by EDI is commonly referred to as type 2 water. To understand the process of EDI, it is necessary to first understand the configuration of a typical EDI module.

EDI modules, or stacks, are comprised of cell pairs. Each cell pair contains an anode on one end and a cathode on the other end (Figure 1). In between the anode and cathode are 3 compartments, all filled with mixed bed ion exchange resins. Each compartment is separated by alternating selectively permeable anion or cation membranes that are manufactured from ion exchange resins. The center compartment is termed the dilute (product water) compartment and the two compartments on either end are termed the concentrating (waste) compartments.

Water enters both the dilute and concentrating compartments. When a direct current (DC) is passed through the cell the cations migrate from the diluting compartment through the cation permeable membrane towards the cathode and the anions migrate from the diluting compartment through the anion permeable membrane towards the anode and both are removed in the concentrating streams. The cations from the left hand concentrate compartment are impeded from passing into the dilute stream towards the cathode by the selectively permeable anion membrane, and likewise in the right hand side concentrate compartment anions are impeded from passing into the dilute stream towards the anode by the selectively permeable cation membrane. The end result is that water passing through the center compartment is deionized in the range of 10-15 MΩ× cm.

The importance of the resin inside the cell pair compartments is that it acts as a conductor, enabling the electrical current to coerce the captured cations and anions through the resin and selectively permeable membranes for concentration and removal in the concentrate streams. This electrical current also serves to split the water molecules into hydrogen and hydroxyl ions in the dilute stream, which act to continuously regenerate the mixed bed resins, ensuring they do not become exhausted and require replacement or regeneration. Splitting occurs only in the dilute stream because as all other charged species are removed and the water is purified, the electrical current is left to act primarily on the water molecule causing splitting to occur.

Electrodeionization as part of a process

As previously mentioned, EDI is a process that acts to remove charged species from a solution, notably water. Though EDI modules remove charged species at an economical and continuous rate, they are somewhat ineffective at removing other species such as bacteria, pyrogens, colloids, particulates, and organics, and thus require appropriate pretreatment steps to perform correctly. A common order of pretreatment steps is illustrated in Figure 2, and includes activated carbon, depth filtration, reverse osmosis and softening.

Figure 2. Schematic of a laboratory EDI system showing the orientation of purification technologies

Activated carbon and a depth filter are included in the pretreatment cartridge in Figure 2. The activated carbon will act to remove chlorine and some organics from water. The importance of removing chlorine is twofold. First it needs to be removed so as not to damage the RO modules which are sensitive to high levels of chlorine. Second, high levels of chlorine will act to oxidize the anion exchange resin within the EDI module, making it more susceptible to scaling. The depth filter will act to remove any particulates or colloids coming in from the general feed water source. The RO modules will act to remove a broad spectrum of contaminants including inorganic and organic contaminants, particulates or colloids to small for the depth filter to remove, and microorganisms and pyrogens. All of these contaminants are removed with efficiency of up to 99%. Finally the softener cartridge will act to remove divalent cations from solution, such as calcium and magnesium. Divalent cations, at high enough concentrations, will cause scaling on the mixed bed resins of the EDI module decreasing the effective lifetime of the module and its performance.

Conclusion

EDI provides water quality in the range of 10-15 MΩ× cm with typically <30 ppb of TOC at high flow rates that can be manipulated by the addition of multiple EDI modules. EDI modules normally only require replacement every 3-5 years, with longer life spans being common. Further, the water from EDI systems can be used for a variety of application, including feeding ultrapure type 1 water systems, feeding instrumentation such as autoclaves and dishwashers, and for general laboratory applications such as buffer and reagent preparation.

About the author

Jamie Grossi is an Associate Product Manager - Water Purification with Sartorius Stedim Biotech. More information about electrodeionization, water purification and related technologies is available from:

Sartorius Stedim Biotech
877-452-2345
www.sartorius-stedim.com