Neptunium has been introduced into the geosphere through nuclear weapons testing and the nuclear fuel cycle. The high toxicity and long half-life of Np237 (t1/2 = 2.14 million years) pose a long term environmental concern. Neptunium is already present in kilogram quantities in the environment and over 50 tons are produced annually by nuclear reactors. Neptunium is also created by Am241 decay (t1/2=432.7 a). For these reasons, neptunium is expected to be a major contributor to the total radiation dose of a nuclear repository after 10,000 years. In aqueous environments, Np(V) exists primarily as NpO2+. Minimal sorption is expected to occur between NpO2+ and most mineral surfaces due to its low effective charge. Therefore, because sorption is an important mechanism responsible for retarding actinide mobility, Np(V) is expected to be highly mobile in the environment. Reactive transport models need to be developed that can accurately predict Np(V) migration for the purpose of risk management. To improve transport models, it is necessary to move away from empirical sorption models and instead develop a better understanding of the molecular level interactions occurring between neptunium and mineral surfaces. Aluminum-bearing minerals, including aluminum (hydr)oxides, account for approximately 8% of the Earth’s crust.1 Because of their abundance, it is important to understand the interactions occurring between actinides and aluminum (hydr)oxides at the water-mineral interface. In this work, neptunium sorption to gibbsite (γ-Al(OH)3), corundum (α-Al2O3), γ-Al2O3 and bayerite (β-Al(OH)3), is investigated. These aluminum (hydr)oxide phases have different surface reactivities,2 which are presumably a function of the amount of aluminum and oxygen exposed to the surface. Even with these different surface reactivities, Kupcik et al. show that similar trivalent actinide/lanthanide sorption behavior is observed on corundum, bayerite and γ-Al2O3.3 However, the relationship between neptunium sorption and aluminum (hydr)oxide surface reactivity has yet to be determined. In the present work, neptunium concentration was kept constant at 10-8 M and sorption was observed as a function of pH (3-10), ionic strength (0.01- 0.1 M NaClO4) and varying mineral concentration (5 -15 m2/L). [1] Krewski et al., J. Toxicol. Environ. Health. 2007, 10, 1-269. [2] Yang et al., W. J. Colloid Interface Sci. 2007, 308, 395-404. [3] Kupcik, et al., J. Colloid Interface Sci. 2015.

Trace-level neptunium (V) sorption to different aluminum (hydr)oxide minerals

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.