Liquid-Solid Separation Techniques

DOE-STD-1128-98

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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