The Radionuclide Rule of the Safe Water Drinking Act requires water systems to monitor for radioactivity in water supplies present as radionuclides. The constituents requiring monitoring include gross alpha emitters, gross beta emitters, radium 226, radium 228 and uranium.
The ruling requires that each wellhead or water introduction point in the system be tested and monitored. The U.S. Environmental Protection Agency has required compliance monitoring through December 2007, with a compliance deadline of January 2008.
Radium is a naturally occurring radioactive substance present in some water supplies. It derives from geological formations and is expected to be found in some groundwater of the coastal Mid-Atlantic states, western states, including California and Colorado, and parts of the Midwest.
Radium Removal by Ion Exchange
A strong acid cation resin in the sodium form can be used to remove radium from water. Radium has a selectivity coefficient of about 40:1 compared with sodium. This is 8.3 times as high as calcium, which is only 5:1.
On a single-use basis, the cation resin will load radium 8.3 times beyond the hardness endpoint. For example, if a resin lasts 1,000 gal per cu ft before hardness starts to break through, it will then last 8,300 gal before radium starts to break through, providing it is only used once. Because of its high selectivity, radium is difficult to remove from the resin. Radium has slow kinetics and is usually loaded at trace quantities, making regeneration even harder.
Sodium Cycle Operation
Termination of the run at the hardness breakthrough. Radium leakages remain consistently low when the cation resins are run only to hardness breakthrough. It is advisable to use regeneration contact times of one hour, regenerant levels of 15 lb per cu ft and salt concentrations of at least 10% at the resin bed.
Termination of the run at radium breakthrough. After a few cycles, this will give only about 5 to 10% more gallonage than hardness break. A normal regeneration level such as 15 to 20 lb of sodium chloride per cu ft at 10 to 15% concentration is not enough to remove all of the radium; therefore, radium will be building up on the resin bed. When the hardness breaks, it will push some of the remaining radium off at the bottom of the bed so that the radium breaks just after hardness. The minimum recommended salt dosage is 15 lb per cu ft at a minimum concentration of 10%.
Calcium Cycle Operation
This procedure leaves the hardness intact and removes only the radium. Because of the high affinity of radium over calcium, it is feasible to remove just radium and leave the hardness intact.
Radium and calcium are both divalent, so concentration itself does not play as important a role in hardness as sodium exchange. Nevertheless, there is a drop in activity of the radium ion in solutions at the higher salt concentrations, so a more concentrated salt produces better results. It is recommended that concentrations of at least 10% calcium chloride be maintained during regeneration.
High initial radium leakages will always be present during cocurrent regeneration unless the resin is mixed after regeneration. Calcium is unable to push off all the radium from the bed but it is more effective than sodium and pushes the radium to the bottom of the bed.
During the subsequent cycle, calcium ions not removed from the solution can act as a continual mild regenerant and push off some of the radium remaining at the bottom of the bed, especially at breakthrough. Mixing the bed after regeneration provides a uniform concentration of radium throughout the bed, which gives consistent and lower leakage.
Another way to use the calcium process effectively is to use countercurrent regeneration (CCR). This way the radium is pushed away from the bottom of the bed so that the radium band is moved up into the bed, thus avoiding the problem of high initial leakage.
No mixing can be used with CCR.
Resin for Single Use
Higher cross-linked strong acid cation resins are best for one-time use rather than multiple cycle use. A 15% cross-linked macroporous resin has about two times the selectivity that an 8% resin has for radium over calcium. During regeneration, however, the radium is even more difficult to remove than from a standard resin. This results in a radium band at the bottom, which makes for higher initial leakages and lower regenerable operating capacities.
The macroporous cation is estimated to have 15 times the selectivity for radium as for hardness, so it can theoretically give 15 times the throughput capacity to a radium break than to a hardness break.
A specialty strong acid cation resin has been developed that effectively targets radium present in water supplies. This resin operates by adsorption of radium using a barium compound that is present in the matrix of the resin bead. It is intended for single use and, under the proper conditions, can generate tremendous throughputs of many gallons per cubic foot. Preliminary pilot tests have shown promising results in a New Jersey installation. There are also some specialty zeolites that have shown favorable selectivities for radium in water.
These specialty media will become more prevalent in the marketplace as municipalities look for ways to meet compliance requirements. The selective resins also have potential for use in removing radium from the used regenerant brine.
Residential systems that are infrequently monitored should include two tanks in a series so that monitoring for radiation can be performed in between the worker tank and the polisher tank. It is also suggested that the radium removal tanks be followed by a carbon filter to capture any radon produced as the radium adsorbed by the resin continues to decay.
Radium removal systems can become radioactive after continued use. It is suggested that any installations be made away from areas where people are likely to spend time, preferably within an enclosure. Exchange of spent resin and tanks should be performed by knowledgeable water treatment professionals wearing appropriate protective gear as necessary.
For radium applications, collect influent water treatment data such as calcium, magnesium, sodium, radium, sulfate, pH and TDS. The sizing of a radium system must be done conservatively to make sure that the resin unit is not overrun. A regenerable resin that is run longer than originally designed can be difficult to regenerate. A once-through resin that is overrun can actually accumulate enough radium to make disposal difficult. In any case, disposal of spent resin and regenerant must be performed in accordance with all local, state and federal regulations.