Ion Exchange

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

8.4.3.4 Ion Exchange

Ion exchange is one of the most useful waste treatment techniques. Aqueous

wastes that are free of oil and other organics and contain only very minimal

quantities of solids may be subject to ion exchange on cation resin, anion resin, or

specialty resins, either alone or in combination. If the contaminant is present as a

cation, such as sodium, ammonia, or calcium, a cation resin can be used to replace

the cation in solution. The cation from the resin will go into the solution to replace

the contaminant cation. If the water stream is being recycled, the cation resin will

probably be in the hydrogen form so that only hydrogen ions will enter the

solution. If a hydrogen form of cation resin is used by itself, the water solution will

likely become more acidic (lower pH). If an anion resin is used, anions in solution

will be replaced with anions from the resin. Although resin may be in a chloride or

other form, the hydroxyl form of the resin is often used so that anions are replaced

with hydroxyl anions (-OH). If only a hydroxyl anion resin is used, the solution

will drop in pH, becoming more basic. If both a hydrogen form of cation resin and

a hydroxyl form of anion resin are used, the ions they add combine to form water,

so both resins are used on demineralized water systems that are recycled. One

disadvantage of most ion-exchange resins for waste treatment is the fact that they

remove all ionic contaminants, not just the radioactive ones, and so are exhausted

earlier than they might be. Selective resins are available for a few materials, most

notably cesium, but are not available for plutonium.

In some applications, radionuclides pass through both cation and anion resin beds.

This is assumed to happen because they are not present in an ionic form. They are

either colloidal or are present in a molecule or complex that is neutral. In these

cases, pretreatment or multiple treatment steps may be required.

Unfortunately, plutonium may be present as a cation, anion, neutral chemical

complex, or colloid. Testing is almost always required to optimize plutonium

removal. One additional limitation in the use of most ion exchange media for

plutonium and other alpha-emitting radionuclides is that the radiation degrades the

resin over time. Organic ion exchange media loaded with large quantities of

plutonium may emit hydrogen and may become unstable when exposed to

oxidizing materials such as nitric acid.

In some applications, ion exchange resins are "recharged" by the addition of large

quantities of a particular ion (e.g., hydrochloric acid may be used to reconvert spent

cation resin to the hydrogen form). In nuclear applications, this is rarely feasible

because of the need to dispose of the recharge solution and because of the large

quantity of rinse water used to remove the excess recharge solution from the resin.

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