All the publicity regarding the nuclear disaster in Japan makes the subject of trace radioactive contaminant removal very topical. However, the trace contaminants radionuclides, cesium, strontium and iodine that are the primary concern from overheated atomic piles and spent fuel are fairly uncommon on a global basis. Far more common are the Naturally Occurring Radioactive Materials, known as NORM, recognized by the U.S. Environmental Protection Agency (EPA).

NORM contaminants include radium, its daughter product radon and uranium. Because uranium and radium are all around us, this first discussion will center on the naturally occurring radioactive contaminants, leaving the unnatural ones for a subsequent discussion.

Naturally occurring contaminants

The naturally occurring slightly radioactive elements thorium and uranium were created in a supernova about seven billion years ago and are about as prevalent in the Earth's crust as lead, their decay product. Thorium forms a tetravalent cation and is quite insoluble — it is never seen as a naturally occurring contaminant. Uranium, on the other hand, forms a divalent or tetravalent anion under most potable water conditions and is sparingly soluble.

Uranium shows up in about one quarter of the country's water supplies. Both uranium and thorium produce ionizing radiation as they decay. The decay chain ends with lead, but includes the intermediate elements radium and radon, which are far more radioactive than their parents. Radium is moderately radioactive and is far more dangerous than U-238. Radium behaves much like the other hardness ions above it in the periodic table. Radon is quite radioactive with a very short half life. Radon is an inert gas and behaves similarly to gasses such as Krypton and Xenon.

The following (abbreviated) table on the top of the next page shows a portion of the decay chain important to water treatment.

Alpha and beta sources

Alpha particles consist of two protons and two neutrons in the form of atomic nuclei. They thus have a positive electrical charge and are emitted from naturally occurring heavy elements such as uranium and radium, as well as from some man-made elements. Because of their relatively large size, alpha particles collide readily with matter and lose their energy quickly. They therefore have little penetrating power and can be stopped by the first layer of skin or a sheet of paper.

However, if alpha sources are taken into the body, for example by breathing or swallowing radioactive dust, alpha particles can affect the body's cells. Inside the body, because they give up their energy over a relatively short distance, alpha particles can inflict more severe biological damage than other radiations.

Beta particles are fast-moving electrons ejected from the nuclei of atoms. These particles are much smaller than alpha particles and can penetrate up to 1 to 2 centimeters of water or human flesh. Beta particles are emitted from many radioactive elements. They can be stopped by a sheet of aluminum a few millimeters thick.

X-rays and gamma rays, like light, represent energy transmitted in a wave without the movement of material, just as heat and light from a fire or the sun travels through space. X-rays and gamma rays are virtually identical except that X-rays are generally produced artificially rather than coming from the atomic nucleus. Unlike light, X-rays and gamma rays have great penetrating power and can pass through the human body. Thick barriers of concrete, lead or water are used as protection from them.

Radiation from natural sources makes up about 85 percent of the dose we receive each year, with almost all the rest coming from medicinal uses. Approximately 45 percent of our average cumulative dose of radiation comes from radon in air, approximately 11 percent comes from naturally occurring radioactive elements found in our drinking water.

Ionizing radiation has been studied over the last 70 years. Without doubt, cumulative doses of ionizing radiation well below what is acutely dangerous have significant adverse long-term health effects leading to cancer and genetic mutations (which might not appear for several generations). Most ionizing radiation is in the form of alpha or beta particles, which do not penetrate very far. The main concern with low concentrations of slightly radioactive contaminants is what happens if they get inside us. Uranium is purged relatively quickly but radium is taken up by bone tissue and bio accumulates; in other words, it stays in the body. When ionizing radiation gets inside a person's body it has nowhere to go, it can't avoid doing damage.

Uranium removal from water

The EPA limit for uranium in water is 30 parts per billion (ppb). Assuming the natural ratio of U-235 to U-238 (about 99.3 percent U-238), most of the radioactivity comes from the small fraction of U-235, the limit of 15 pCi/L gross alpha is somewhat more stringent than the concentration limit. In addition to the general limit of 2 nanocurries per gram maximum for low level waste there is a secondary limit of 0.05 percent by weight. Higher concentrations require registration with the NRC. In most but not all cases, a strong base anion resin will concentrate uranium well beyond the 0.05 percent rule.

Uranium forms a complex anion in the presence of alkalinity. Since almost all potable water supplies contain at least most amounts of alkalinity, uranium is almost always present as a complex anion, uranyl carbonate. At lower pH the complex is divalent but at neutral pH and above the complex is tetravalent.

The selectivity constant for uranium (the "K" value) is quite small (less than 0.1 compared to chloride). Using chloride as the reference ion, the apparent selectivity for ions with valence greater than one is concentration dependent. This means that the uranium anion is greatly preferred at low TDS. For waters less than 100 ppm, TDS and with little or no sulfate, uranium can concentrate to more than 75 percent of the resin's capacity. The major study regarding uranium sponsored by the EPA demonstrated that thruputs greater than 100,000 bed volumes could be obtained in low TDS waters that contained little or no sulfate. As TDS and/or sulfate increase, anion resin capacity for uranium steadily decreases. At about 1,000 ppm TDS and 250 ppm sulfate, a resin's capacity for uranium decreases to the point where the 0.05 percent rule is no longer an issue, uranium capacity becomes a very small fraction of the total resin's capacity.

The same relationship between capacity and TDS makes a uranium laden resin fairly easy to regenerate. A relatively high concentration of salt reverses the selectivity and allows uranium to be readily displaced from the resin. However, as uranium is a very large and heavy ion, the elution of uranium from a resin is very much time related. Regeneration is also complicated by a high probability of resin fouling.

Strong base anion resins remove many species besides chloride, sulfate and uranium. They also remove alkalinity, although not very well, and Naturally occurring Organic Matter (NOM). This makes the resins used for uranium removal susceptible to fouling over time. Indeed, few uranium removal systems ever make it to the promise of 100,000+ bed volumes of thruput because they almost invariably foul first.

Radium removal

All strong acid cation resins, including those used for water softening have very high preference for radium ions. So long as a system is removing hardness, radium is almost completely removed. A relatively inexpensive method of removing radium is to soften the water and a garden variety water softener is admirably suited for this purpose.

However, if a water softener is overrun past hardness break, it may eventually fail to remove radium. Radium won't spike but might not be completely removed. And for larger systems the cost of salt may be prohibitive. There are several options for single use radium removal resins.

Selectivity is related to swelling pressure, which is in turn related to how much divinylbenzene crosslinking a resin has. The highly crosslinked macroporous cation resins have markedly increased selectivity for radium over other hardness ions. This is of little help in a water softener but opens the door for single use resins whenever the TDS and hardness levels are low or moderate.

A radium selective hybrid ion exchanger was invented back in the 1970s and has seen some use in potable water systems. It is a radium complexer resin, a cation resin similar to a water softening resin that has barium sulfate precipitated inside of it. Radium is removed first by ion exchange and then replaces barium in the precipitant, according to the rule of "least soluble salt." The complexer resin will concentrate radium well beyond what a cation resin alone will do, sometimes to the point where the resin becomes significantly radioactive and dangerous to handle.

The complexer resin is not without potential drawbacks. The barium sulfate inside the resin is not 100 percent stable and can leave the precipitant, move back onto the exchange sites and then be exchanged out of the resin for other hardness ions such as calcium. For this reason the radium complexer resin may not be suitable for waters that do not contain at least some sulfate ion (sulfate ion prevents barium from escaping the resin).

The complexer resin can hold enough radium that there is an increase in alpha radiation caused by formation of radium's daughter product radon. It is not unusual for gross alpha to decrease, then slowly increase as radon formation increases in the resin.

Radon

Although radon is not ionized, it can be removed by several different water treatment methods. Granular carbon has the general ability to adsorb radon and hold it long enough for it to decay (after all the half life of radon is less than four days). General sizing should be no more than 1.5 gpm/cu ft. (>5 minute EBCT). Granular carbon holds the radon while it decays into more stable elements, such as lead. However, the carbon filter can become substantially radioactive during use, enough that careful placement of the filter, well away from living spaces, is recommended. The alternative is aeration, a process where radon is striped from the water into air, the air is then vented in a way that dissipates the radon.

Disposal of slightly radioactive spent media

Disposal of slightly radioactive medias used to remove naturally occurring radioactive materials is known as TENORM (Technologically Enhanced Naturally Occurring Radioactive Materials) waste. Many states now have disposal sites that will accept TENORM waste up to approximately 1,800 pico curies per gram of radiation. To avoid issues with waste disposal, many states (including New Jersey) recommend the use of systems that are regenerated as opposed to single use.

This ends the discussion on NORM type radioactive contaminants. A future article will discuss the most likely contaminants to escape from nuclear power plants.

Peter Meyers is technical director for ResinTech Inc. of West Berlin, N.J. He has more than 35 years of experience covering a wide range of ion exchange applications from demineralizers, polishers and softeners to industrial process design and operation.