Several techniques for Radium removal from water

By Magazine Team Last updated Jun 2, 2025 433 0

Contents

1 Ion Exchange

2 Co-Precipitation With HMO

Radium removal from water is a crucial issue as it is known to cause cancer. U.S. EPA created the Radionuclides Rule, which limits the combined levels of radium 226/228 to a Gross Alpha of 15 picoCuries per liter and a Maximum Contaminant Level (MCL) of 5 picoCuries per liter.

Radium is commonly found in water as different combinations of three isotopes: Ra224, Ra226 and Ra228. However, there are 25 known isotopes of radium. Ra224 adds to the Gross Alpha if a sample is examined right away after collection, despite its quick disintegration and short half-life of 3.66 days. With a half-life of 1,599 years and a half-life of 5.76 years, Ra226 and Ra228 are more concerning.

Ion Exchange

Radium can effectively be removed from drinking water by ion exchange softening. The radium is exchanged along with the calcium and magnesium hardness. In the ion exchange softening process the elements removed from the water are replaced by sodium. The ions which are removed are washed from the resin using a brine solution.

Regeneration of the resin by means of a brine wash is required when the resin becomes saturated with the elements being removed, since it is no longer effective. For effective radium removal the resin must be regenerated as soon as calcium breakthrough occurs.

Since ion exchange softening removes all the hardness, it is necessary to blend the finished water or to treat the finished water to prevent corrosion of the distribution system. Blending of finished water with raw water will result in decreased cost of removal. The ratio of raw to finished water which can be used will be dependent on the level of radium in the raw water.

Co-Precipitation With HMO

Iron and manganese are usually removed using greensand or other manganese dioxide media filters, although it was noted many years ago that some radium might also be removed by similar filters. It is feasible to co-precipitate radium during manganese removal by adding Hydrous Manganese Oxide (HMO), as radium forms a strong connection with manganese dioxide.

Water is mixed with preformed manganese oxide and filtered using a conventional iron and manganese removal technique during HMO co-precipitation. Backwashing the filter removes radium as a solid from the water. Hydrous manganese oxide filters or lime softening filters can be used in co-precipitation systems. The removal of iron and manganese is an extra advantage of these systems.

Lime Softening

Lime softening is reported to be as effective as ion exchange for the removal of radium. The radium is precipitated and carried down with the calcium during softening. Since complete softening of the water does not occur, no blending would be required. However, depending on the level of radium in the raw water, blending may be used to reduce the amount of water which is treated.

The lime softening process does not require as close monitoring as the ion exchange since no regeneration is required. There may be some limitations on the process for high levels of radium and low hardness.

Reverse Osmosis

Reverse osmosis involves the removal of soluble minerals by passage of water through a semipermeable membrane. To get water to pass through the membrane , pressure should be applied to the water containing the minerals to overcome the natural direction of flow which would be for pure water to diffuse into the mineral containing water.

The amount of pressure necessary is dependent on the mineral content of the raw water. Although reverse osmosis can be used to reduce the radium level, its application is impractical and costly unless it is already in use for the treatment of brackish water.

The most significant cost is plant construction. For a 1000 m³/day plant (183 gpm), cost will be about $250,000 based on 1976 costs. This cost does not include costs for any interest during construction, site and site improvement, discharge facilities, storage and delivery facilities, or special treatment. Operating costs are about $18,000 for a plant of that capacity.

Electrodialysis

Electrodialysis involves the removal of salts by means of ion selective membranes and a d.c. current to assist transport of the ions across the membrane. There is depletion of ions on one side of the membrane if current is passed for any length of time, while there is concentration on the other side of the membrane. Any level of desalting can be achieved by increasing the residence time or increasing the current density.

For efficient operation good water pretreatment is required This should include coagulation of colloidal particles, oxidation of iron and soluble organics, carbon filtration, and finally acidi fication.

Although this process can be used for the reduction of radium levels, its application is impractical and costly even if other contaminants are to be removed unless the equipment is already in use or planned for use to reduce brackish water to an acceptable salt level.

The cost for electrodialysis is dependent on the level of contaminant to be reduced. In general it will be more costly than reverse osmosis. The pH of the effluent may require adjustment to protect the distribution system.

Distillation

Distillation involves the volatilization of water to separate it from all dissolved or suspended materials which are not volatilized. Normally the water is heated under pressure to improve the thermal efficiency of the method by recovering some of the heat.

This process produces water of very low dissolved solids. Since the water is corrosive to the distribution system, it is necessary to increase the salt content. This can normally be accomplished by appropriate blending of the finished water and the raw water. Some pretreatment of the feed-water may be necessary. Most often only deaeration is necessary, but in some situations it may be necessary to remove suspended solids and calcium and magnesium to prevent scaling.

Distillation is a relatively expensive and impractical solution for the removal of specific contaminants from water. The process involves the removal of a large volume of water from a small amount of dissolved material.

This results in an unfavorable energy requirement since it is essentially independent of the contaminant level and only dependent on the amount of water to be treated. The major cost is plant construction which will be about $1.2 million for a 1000 m3/day plant (183 gpm) . The operating costs for energy are also high, since there is only partial heat recovery in this process.

Conclusion

Radium removal from water can be achieved using any of the techniques outlined in this article, and each one concentrates the radioactive element in either liquid or solid waste. The process of choosing the best removal technique is usually site-specific and focuses on capital and operating costs, the existence of other contaminants, and waste disposal concerns.

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