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Ion Exchange Method
Date:2013-07-31

 Ion Exchange
Ion exchange is widely used in laboratories for providing purified water on demand. Laboratory deionizers invariably incorporate mixed-bed cartridges of ion exchange resins that are either returned to a regeneration station for recharging when they become exhausted, or else discarded.


Anions and cations in the feed water are removed by the ion exchange resins and are replaced by hydrogen and hydroxyl ions from the resin. The hydrogen and hydroxyl ions combine to form water molecules.


RephiLe’s cartridges filled with LeFil™ and OrganeFil™ media. Those media are composed of nuclear grade ion exchange resins and nuclear grade activated carbon to remove specific types of water contaminants.


 How does ion exchange work?
Ion exchange functions by exchanging hydrogen ions for cationic contaminants and hydroxyl ions for anionic contaminants in the feed water. The ion exchange resin beds are made up of tiny spherically shaped beads through which the feed water passes. After a period of time, cations and anions will have replaced most of the active hydrogen and hydroxyl sites in the resins and the cartridges will need to be replaced or regenerated.


 Ion-Exchange Resin Structure:

 


 Ion-Exchange Processes: Mixed Bed

 

What are the benefits of ion exchange?
Ion exchange has many advantages over distillation for the production of purified water. First of all, it is an on-demand process; the water is available when it is needed. Secondly, when using high purity resin materials, effectively all the ionic material will be removed from the water to give a maximum resistivity of 18.2 MΩ-cm (at 25ºC). Tiny fragments of the ion exchange resin materials can be washed out of the cartridge by the water passing through it. Therefore, ion exchange should be used in conjunction with filters if particle-free water is required. As bacteria will grow rapidly in stagnant water, the cartridges may become contaminated if they are not regularly used. The problem is alleviated by frequent recirculation of the water to inhibit bacterial build up and by regular replacement or regeneration of the resins, since regenerant chemicals are powerful disinfectants.


 What are the limitations of ion exchange?
Ion exchange will only remove polar organic compounds from the water and dissolved organics can foul the ion exchange beads, decreasing their capacity. Where organically and inorganically pure water is needed, the combination of reverse osmosis followed by ion exchange is particularly effective.


There have been many attempts to overcome some of the limitations of ion exchange and distillation. In some systems distillation has preceded ion exchange - the cartridges last much longer, but the problem of bacteria remains. In some others, ion exchange has preceded distillation - but then the problems of storage and not having water on demand remain.

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