Answer to Question #10438 Submitted to "Ask the Experts"

My company has been asked to design a water treatment filter to remove radium from groundwater that will be used for public consumption. The filter will be located within a carbon steel pressure vessel. During filter backwash, the radium concentration will reach 1.347 x 10^_-2 Bq cm^-3. Will the radium accumulate on the walls of the vessel? If so, can you please point me to a resource that will help me quantify the rate of accumulation.

The extent of radium plateout on the walls of the steel pressure vessel during the backwash process will depend on the residence time of the backwash water in the vessel and the presence of various chemical species either in the water or added during the backwash cycle. The presence of a sulfate anion may be important because it forms a quite insoluble compound with Ra⁺⁺ which may encourage the coprecipitation of the radium with other species if present; trace amounts of barium are especially relevant in this regard, but coprecipitation may also occur with oxides of iron and manganese and with calcium salts that might be present. Because I would expect the residence time of the backwash solution(s) to be quite short, I suspect that the accumulation of radium that has been released from the filter and transferred to the wash water during this process could be kept to a small amount, especially if you can control the chemistry of the backwash.

I would pay at least some additional attention, however, to the accumulation of radium on the walls of the vessel and associated components during routine operation. The coprecipitation of radium with salts of elements such as iron, manganese, and calcium can lead to elevated radium concentrations in scale deposited on the steel surfaces. The enhanced radium concentrations in the scale can lead to releases of elevated radon levels that can impact drinking water quality. This phenomenon has been observed in cases in which pipes supplying domestic water have built up radium-containing scale that resulted in waterborne radon levels greatly in excess of those observed at the source of the water supply (see, for example, see this brief 1995 article by R.W. Field et al.) The available surface-to-volume ratio of the pressure vessel and associated plumbing plays an important part in the significance of this phenomenon, and I do not know whether it will be important in your case.

I would expect that you will probably have a scheduled program for controlling buildup of and/or removing scale from the vessel walls as appropriate. The presence of other cations and anions, the deposition surface characteristics, the water pH, pressure and temperature, and other factors affect the extent and rate of radium deposition. Unfortunately, I am not aware of any information that relates the many variables involved in the extent of radium plateout on steel surfaces with time, and I cannot provide any accumulation rate information.

I wish you well in your endeavors.

George Chabot, PhD