Ion Exchange Technologies

Ion-exchange is one of the most common and attractive treatment methods for liquid radioactive waste [6.21]. It is a well developed technique that has been employed for many years in both nuclear and non-nuclear industries. Organic ion exchange resins are typically used to control the water systems chemistry at NPPs for minimizing corrosion or degradation of system components and to remove radioactive contaminants. Organic resins are also used in a number of chemical decontamination or cleaning processes for regeneration of process water by reagent and radionuclide removal.

In the past decade, inorganic ion exchange materials have emerged as an increasingly important tool to replace or complement conventional organic ion exchange resins, particularly in liquid radioactive waste treatment and spent fuel reprocessing applications. Inorganic ion exchangers often have the advantage of much greater selectivity than organic resins for certain radiologically important species such as cesium and strontium.

Although inorganic ion exchange materials are playing an increasing role in the selective removal of specific radionuclides, organic ion exchange resins continue, globally, to represent the dominant form of ion exchange waste material. This is largely because organic ion exchange has been proven to be reliable and effective for control of both the chemistry and radiochemistry of water coolant systems at NPP and also for processing some liquid radioactive waste. In a number of cases, the organic resins cannot be replaced by inorganic ion exchangers for specific physical and chemical reasons.

A general comparison between organic and inorganic ion exchangers is given Table 6-1 [6.21].

Following the table is an overview of the factors to consider for ion exchange.

Table 6-1: General Comparison of Organic and Inorganic Ion Exchangers Organic

exchangers Inorganic exchangers

Comments

Thermal stability fair-poor good Inorganic specially good for long term

Chemical stability good fair to good Specific organics and inorganics are available for any given pH range

Radiation stability fair-poor good Organics very poor in combination with high temperature and oxygen

Exchange

capacity high low to high Exchange capacity will be a function of nature of the ion being removed, its chemical environment and the experimental conditions

Selectivity available available For some applications, such as cesium removal, inorganics can be much better than organics due to greater selectivity. Ion selective media are available in both organic and inorganic forms

Regeneration good uncertain Most inorganics are sorption based which limits regeneration

Mechanical

strength good variable Inorganics may be brittle or soft or may breakdown outside limited pH range

Cost medium to

high low to high more common inorganics are less costly than organics Availability good good Both types are available from a number of commercial

sources

Immobilization good good Inorganics can be converted to equivalent mineral structures, organics can be immobilized in a variety of matrices, or can be incinerated

Handling good fair Organics are generally tough spheres, inorganics may be brittle and angular particles are more friable

Ease of use good good If available in a granulated form both types are easy to use in batch or column applications

Factors to consider for applying ion exchange to process radioactive waste Waste Characteristics

Ion exchange is generally suited for treatment of aqueous liquids satisfying the following criteria:

• For column use with downward flow of liquid, the concentration of total suspended solids in the waste should be low, normally less than 4 mg/L, to prevent fouling of the ion exchange media by the solids filtered out by the ion exchanger bed.

• The waste should have a low total dissolved salts (TDS) content, normally less than 1 to 2 g/L, to prevent rapid exhaustion of the ion exchange capacity. When highly selective ion exchange materials are used the TDS can be up to several hundreds of g/L.

• Radionuclides should be present in a suitable ionic form. Adjustment of the solution pH is often sufficient to convert a radionuclide into species that can be removed by ion exchange.

• Generally the waste should contain only very small amounts of organic contaminants.

Some compounds, such as oils and greases, if present as a separate phase, will result in severe fouling of the ion exchange media and loss of ion exchange capacity. Others,

such as certain organic solvents, may cause physical decomposition of organic ion exchange media.

Selection of an ion exchanger and a treatment process

A wide range of ion exchange media is now available, from low cost naturally occurring organics (such as coals and peat) and inorganics (such as clays and natural zeolites) to expensive synthetic organics and inorganics engineered to remove specific ions.

The selection of appropriate media depends on the needs of the system. Higher cost ion specific exchangers may be a better choice if the extra cost of the media is more than offset by the reduction in the total cost for treatment and subsequent storage/disposal of the spent media.

The ion exchange media must be compatible with the chemical nature of the waste (such as pH and the type of ionic species present), as well as the operating parameters, notably temperature and pressure.

The total radioactivity content of the waste should be considered in the selection of the ion exchange process. Concentration of the radioactivity onto the ion exchanger can greatly increase (by many orders of magnitude) the radiation fields surrounding the ion exchange. The radiation levels can also affect the stability of the ion exchange media, which is a concern for long term storage and disposal.

Ion exchange application techniques

Ion exchange processes can be implemented in a variety of ways, including batch operation, columns, continuous loops, and as part of, or in combination with, membrane processes. Each technique has features that make it more or less suitable for specific applications [6.21].

Ion exchange regeneration options

In theory, the ion exchange process is reversible. Most ion exchange media can be regenerated by using an appropriate strong acid (such as HNO₃) for cation media or alkali (such as NaOH) for anion media, to replace the bound contaminant ions on the media and restore the media to its original chemical form. The life of media can be extended, by regeneration, thus saving on costs of new media and disposal of old media. The fact that regeneration does typically restore ion exchange capabilities only to 90% is an important factor for nuclear grade ion exchangers, because nuclear grade quality is maintained only until the first regeneration. On top of that, regeneration is not practiced for radioactive systems since the concentrated acid and caustic regeneration solutions are often more difficult and costly to treat than the money saved by lower media usage.

Integration with other waste treatment systems

Ion exchange is frequently employed in combination with other liquid treatment processes, usually as a final or “polishing” step. For example, conventional filtration can be used to remove particulate

“crud”, and the filtrate then treated by ion exchange. Total dissolved solids may be reduced by chemical precipitation and the clarified liquid can then be polished by ion exchange.

Management of spent ion exchange materials

Spent ion exchange materials represent a special type of radioactive waste, posing unique problems in the selection of treatment options since they often contain high concentrations of radioactivity because of the function that they fulfil. In the past, these materials were often disposed of in drums or boxes or as disposable ion-exchange columns without treatment. In some operations, the resins were sluiced from columns and stored in underground tanks as a bed settled in water, pending future disposition.

Selection of treatment options for spent ion exchange materials must consider the physical and chemical characteristics of the material. Basically, there are two main methods for treatment of spent organic ion exchange materials: (1) destruction of the organic compounds to produce an inorganic

intermediate product that may or may not be further conditioned for storage and/or disposal and (2) direct immobilization producing a stable end product.

The immobilization matrices currently used for spent ion exchangers are cement, bitumen and some polymers. In some countries “high integrity containers” are used for storage and disposal of spent ion exchange media, without incorporation into a solidification matrix. Most of the processes using the above matrices are performed on an industrial scale and are described in detail in many publications.

Table 6-2 summarizes the general advantages and limitations of these immobilization matrixes [6.21].

It should be noted that the specific application of a matrix for a given waste must be evaluated on an individual basis. Pilot level studies may be required to determine the optimum waste loading and matrix composition. This is especially true for cements and some polymers, where trace amounts of certain materials within the wastes may adversely affect the properties of the product (for example, preventing the cement or polymer from solidifying, or producing a product of poor quality).

Table 6-2: Summary Comparison of Immobilization Processes

Matrix Advantages Disadvantages

Cement - Material is readily available and not expensive;

- Compatible with wide range of materials;

- Excellent radiation stability;

- Non flammable product;

- High pH results in good chemical retention most of radionuclides.

- Swelling of organic bead resins may cause cracking of matrix;

- Waste loading can be low, volume of final waste form is greater than original waste volume;

- Moderate leach resistance for many radionuclides, such as Cs.

Bitumen - Good leach resistance;

- All water in waste is removed by process resulting in good waste loadings.

- Waste form will soften at moderate temperatures;

- Requires container to maintain structural stability;

- Organic bead resins may swell and compromise waste form if prolonged contact with water;

- Organic waste form may be flammable and subject to biodegradation;

- Lower radiation stability than cement.

Polymers - Wide variety of polymers available;

- Good leach resistance for many polymers;

- Generally more expensive than bitumen or cement;

- Polymerization reaction can be affected by trace materials in the wastes;

- Lower radiation stability than cement.

High integrity

container - Simple and inexpensive to operate and handle;

- Steel containers have excellent radiation stability.

- Relies entirely on container integrity;

- Not accepted in all jurisdictions;

- Polymer containers can have low radiation stability.

Vitrification - Glass waste form has excellent radiation stability and leach resistance;

- Good volume reduction in process.

- High temperature process;

- Expensive to operate.

In addition to the technical factors outlined above, other factors, such as national standards and waste acceptance criteria for storage and/or disposal facilities, must also be considered. The evolution of performance-based disposal facility acceptance criteria now requires that spent ion exchange materials meet specific quality requirements prior to disposal. Where disposal facilities exist, waste acceptance criteria define the quality of waste forms for disposal and, therefore, will sometimes define appropriate treatment options.