Precipitation, filtration, and ion exchange are widely used treatment technologies for radionuclides in ground water and are discussed in the following paragraphs.
The combination of precipitation/flocculation and sedimentation is a well-established technology for radionuclides removal from ground water. This technology pumps ground water through extraction wells and then treats it to precipitate radionuclides and heavy metals. Typical removal of radionuclides employs precipitation with hydroxides, carbonates, or sulfides. Hydroxide precipitation with lime or sodium hydroxide is the most common choice. Generally, the precipitating agent is added to water in a rapid-mixing tank along with flocculating agents such as alum, lime, and/or various iron salts. This mixture then flows to a flocculation chamber that agglomerates particles, which are then separated from the liquid phase in a sedimentation chamber. Other physical processes, such as filtration, may follow.
Filtration isolates solid particles by running a fluid stream through a porous medium. The driving force is either gravity or a pressure differential across the filtration medium. Pressure differentiated filtration techniques include separation by centrifugal force, vacuum, or positive pressure. The chemicals are not destroyed; they are merely concentrated, making reclamation possible. Parallel installation of double filters is recommended so ground water extraction or injection pumps do not have to stop operating when filters backwashed.
Ion exchange is a process whereby the toxic ions are removed from the aqueous phase in an exchange with relatively innocuous ions (e.g., NaCl) held by the ion exchange material. Modern ion exchange resins consist of synthetic organic materials containing ionic functional groups to which exchangeable ions are attached. These synthetic resins are structurally stable and exhibit a high exchange capacity. They can be tailored to show selectivity towards specific ions. The exchange reaction is reversible and concentration dependent; the exchange resins are regenerable for reuse. The regeneration step leads to a 2 to10% wastestream that must be treated separately.