Selection and Testing of "Getters" for Adsorption of Iodine-129 and Technetium-99: A Review
During the last several decades considerable research effort has been expended to identify suitable "getter" materials that can immobilize or delay the transport of anionic radionuclides (such as 129I and 99TcO4) that would be released from physically and chemically degrading waste packages. Several investigators have identified a number of important performance characteristics such as adsorption potential and chemical/physical stability that getter materials should possess for effective deployment in repository environments. A review of published literature indicated that various minerals and synthetic adsorbent materials such as, oxides, hydroxides, natural and modified aluminosilicates, sulfides, carbonates, phosphates, carbon, and functionalized novel sorbents have been tested for their getter properties. Oxide/hydroxide solids and many silicate minerals have poor capacity and selectivity for iodide (Kd : 0 – 10 ml/g) and release iodide with increasing pH. A few silicate minerals such as illite exhibit better selectivity for iodide (Kd : ~30 ml/g). Significantly improved iodide selectivity (Kd : 5,000 ml/g) has been observed with organically-modified silicate minerals such as montmorillonite and vermiculite. Sulfide minerals such as cinnabar and argentite typically adsorb iodide with very high selectivity (Kd : 3,000 – 80,000 ml/g). Synthetic materials such as calcium monosulfate aluminate and hydrotalcite show moderate iodide selectivity (Kd: ~25 – 300 ml/g). A new class of specially-designed synthetic getters when tested in groundwater and simulated waste package leachate adsorbed iodide with very high specificity (Kd: 100,000 – 1,000,000 ml/g). The long-term stability of these mineral and synthetic getters in post-closure environment needs further evaluation. Under oxic conditions and in groundwater or background salt solution, many of the oxide/ hydroxide solids and silicate minerals exhibit relatively poor capacity and selectivity (Kd: <5 ml/g) for technetium. Therefore, a potential lack of long-term stability preclude using many of the oxide and hydroxide minerals as effective getter materials for technetium. Although, magnetite and metallic iron under certain limited conditions appear to be effective getters for Tc (Kd: 350 – 122,000 ml/g), their chemical stability and their tendency for Tc mobilization from radiolysis effects and potential influx of oxic ground waters and/or leachates need to be evaluated on a site-specific basis. Among sulfide minerals, greigite appears to be best in immobilizing Tc (Kd: ~110 – 6100 ml/g). Principal mechanisms for Tc attenuation by sulfide minerals are through either reduction of Tc(VII) to low solubility Tc(IV) phases such as TcO2•xH2O or by direct precipitation of Tc(VII) as a sparingly-soluble sulfide mineral, Tc2S7. Novel Cu-EDA SAMMS getters in groundwater and simulated waste package leachate have been shown to adsorb TcO4- (Kd: ~50 – 800 ml/g). The long-term stabilities and the adsorption performance of miscellaneous minerals and synthetic compounds under typical post closure conditions in any facility such as the Immobilized Low Activity Waste (ILAW) repository at Hanford need further evaluation. A selection and evaluation scheme is proposed to identify appropriate getter materials that are i) chemically (including redox) and physically stable enough to be used as barrier materials, ii) not affected adversely by biotic factors that would compromise the contaminant adsorption performance to any measurable degree, and iii) cost effective and offer flexibility in deployment.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15004678
- Report Number(s):
- PNNL-14208; 820201000; TRN: US200320%%307
- Country of Publication:
- United States
- Language:
- English
Similar Records
Novel Functionalized Ceramic Getter Materials for Adsorption of Radioiodine
Technetium and Iodine Getters to Improve Cast Stone Performance